Help: Better explain CMAKE_<LANG>_EXTENSIONS_DEFAULT

Explain that this represents the compiler's default and mustn't be modified
by the user. Clarify when it's used as the default.

Additionally:
* Add a reference to it in cmake-compile-features in text explaining the
  feature.
* Add explanations for the default initialization by
  `CMAKE_<LANG>_EXTENSIONS_DEFAULT` to all `<LANG>_EXTENSIONS` pages and
  references to CMP0128.
* Slightly reduce the wordiness of the default initialization explanations by
  removing an unnecessary "it is".

Fixes #22828.

7 files changed, 26 insertions, 13 deletions

diff --git a/Help/prop_tgt/CUDA_EXTENSIONS.rst b/Help/prop_tgt/CUDA_EXTENSIONS.rst
index 2ddba0b..ab467ac 100644
--- a/Help/prop_tgt/CUDA_EXTENSIONS.rst
+++ b/Help/prop_tgt/CUDA_EXTENSIONS.rst
@@ -15,5 +15,7 @@ See the :manual:`cmake-compile-features(7)` manual for information on
 compile features and a list of supported compilers.
 
 This property is initialized by the value of
-the :variable:`CMAKE_CUDA_EXTENSIONS` variable if it is set when a target
-is created.
+the :variable:`CMAKE_CUDA_EXTENSIONS` variable if set when a target is created
+and otherwise by the value of
+:variable:`CMAKE_CUDA_EXTENSIONS_DEFAULT <CMAKE_<LANG>_EXTENSIONS_DEFAULT>`
+(see :policy:`CMP0128`).
diff --git a/Help/prop_tgt/CXX_EXTENSIONS.rst b/Help/prop_tgt/CXX_EXTENSIONS.rst
index bda531e..9b52504 100644
--- a/Help/prop_tgt/CXX_EXTENSIONS.rst
+++ b/Help/prop_tgt/CXX_EXTENSIONS.rst
@@ -15,5 +15,7 @@ See the :manual:`cmake-compile-features(7)` manual for information on
 compile features and a list of supported compilers.
 
 This property is initialized by the value of
-the :variable:`CMAKE_CXX_EXTENSIONS` variable if it is set when a target
-is created.
+the :variable:`CMAKE_CXX_EXTENSIONS` variable if set when a target is created
+and otherwise by the value of
+:variable:`CMAKE_CXX_EXTENSIONS_DEFAULT <CMAKE_<LANG>_EXTENSIONS_DEFAULT>` (see
+:policy:`CMP0128`).
diff --git a/Help/prop_tgt/C_EXTENSIONS.rst b/Help/prop_tgt/C_EXTENSIONS.rst
index b2abb46..a53090c 100644
--- a/Help/prop_tgt/C_EXTENSIONS.rst
+++ b/Help/prop_tgt/C_EXTENSIONS.rst
@@ -15,5 +15,7 @@ See the :manual:`cmake-compile-features(7)` manual for information on
 compile features and a list of supported compilers.
 
 This property is initialized by the value of
-the :variable:`CMAKE_C_EXTENSIONS` variable if it is set when a target
-is created.
+the :variable:`CMAKE_C_EXTENSIONS` variable if set when a target is created and
+otherwise by the value of
+:variable:`CMAKE_C_EXTENSIONS_DEFAULT <CMAKE_<LANG>_EXTENSIONS_DEFAULT>` (see
+:policy:`CMP0128`).
diff --git a/Help/prop_tgt/HIP_EXTENSIONS.rst b/Help/prop_tgt/HIP_EXTENSIONS.rst
index b731753..d1475c9 100644
--- a/Help/prop_tgt/HIP_EXTENSIONS.rst
+++ b/Help/prop_tgt/HIP_EXTENSIONS.rst
@@ -15,5 +15,7 @@ See the :manual:`cmake-compile-features(7)` manual for information on
 compile features and a list of supported compilers.
 
 This property is initialized by the value of
-the :variable:`CMAKE_HIP_EXTENSIONS` variable if it is set when a target
-is created.
+the :variable:`CMAKE_HIP_EXTENSIONS` variable if set when a target is created
+and otherwise by the value of
+:variable:`CMAKE_HIP_EXTENSIONS_DEFAULT <CMAKE_<LANG>_EXTENSIONS_DEFAULT>` (see
+:policy:`CMP0128`).
diff --git a/Help/prop_tgt/LANG_EXTENSIONS.rst b/Help/prop_tgt/LANG_EXTENSIONS.rst
index 2add21d..afc2ad3 100644
--- a/Help/prop_tgt/LANG_EXTENSIONS.rst
+++ b/Help/prop_tgt/LANG_EXTENSIONS.rst
@@ -14,7 +14,8 @@ These properties specify whether compiler-specific extensions are requested.
 
 These properties are initialized by the value of the
 :variable:`CMAKE_<LANG>_EXTENSIONS` variable if it is set when a target is
-created.
+created and otherwise by the value of
+:variable:`CMAKE_<LANG>_EXTENSIONS_DEFAULT` (see :policy:`CMP0128`).
 
 For supported CMake versions see the respective pages.
 To control language standard versions see :prop_tgt:`<LANG>_STANDARD`.
diff --git a/Help/prop_tgt/OBJCXX_EXTENSIONS.rst b/Help/prop_tgt/OBJCXX_EXTENSIONS.rst
index 8a254f2..2a15dec 100644
--- a/Help/prop_tgt/OBJCXX_EXTENSIONS.rst
+++ b/Help/prop_tgt/OBJCXX_EXTENSIONS.rst
@@ -18,5 +18,7 @@ If the property is not set, and the project has set the :prop_tgt:`CXX_EXTENSION
 the value of :prop_tgt:`CXX_EXTENSIONS` is set for :prop_tgt:`OBJCXX_EXTENSIONS`.
 
 This property is initialized by the value of
-the :variable:`CMAKE_OBJCXX_EXTENSIONS` variable if it is set when a target
-is created.
+the :variable:`CMAKE_OBJCXX_EXTENSIONS` variable if set when a target is
+created and otherwise by the value of
+:variable:`CMAKE_OBJCXX_EXTENSIONS_DEFAULT <CMAKE_<LANG>_EXTENSIONS_DEFAULT>`
+(see :policy:`CMP0128`).
diff --git a/Help/prop_tgt/OBJC_EXTENSIONS.rst b/Help/prop_tgt/OBJC_EXTENSIONS.rst
index ef1c754..cd72e5f 100644
--- a/Help/prop_tgt/OBJC_EXTENSIONS.rst
+++ b/Help/prop_tgt/OBJC_EXTENSIONS.rst
@@ -18,5 +18,7 @@ See the :manual:`cmake-compile-features(7)` manual for information on
 compile features and a list of supported compilers.
 
 This property is initialized by the value of
-the :variable:`CMAKE_OBJC_EXTENSIONS` variable if it is set when a target
-is created.
+the :variable:`CMAKE_OBJC_EXTENSIONS` variable if set when a target is created
+and otherwise by the value of
+:variable:`CMAKE_OBJC_EXTENSIONS_DEFAULT <CMAKE_<LANG>_EXTENSIONS_DEFAULT>`
+(see :policy:`CMP0128`).