Help: Clarify that set(VAR) unsets VAR

Also call out possible exposure of an existing cache entry of the same
name, as we have for the `unset` command since commit 1a0f1a25a4 (Help:
Clarify the unset command, 2018-08-17, v3.13.0-rc1~199^2).

Issue: #24633

1 files changed, 3 insertions, 8 deletions

diff --git a/Help/command/unset.rst b/Help/command/unset.rst
index 1cd1398..522be89 100644
--- a/Help/command/unset.rst
+++ b/Help/command/unset.rst
@@ -12,19 +12,14 @@ Unset Normal Variable or Cache Entry
 
 Removes a normal variable from the current scope, causing it
 to become undefined.  If ``CACHE`` is present, then a cache variable
-is removed instead of a normal variable.  Note that when evaluating
-:ref:`Variable References` of the form ``${VAR}``, CMake first searches
-for a normal variable with that name.  If no such normal variable exists,
-CMake will then search for a cache entry with that name.  Because of this
-unsetting a normal variable can expose a cache variable that was previously
-hidden.  To force a variable reference of the form ``${VAR}`` to return an
-empty string, use ``set(<variable> "")``, which clears the normal variable
-but leaves it defined.
+is removed instead of a normal variable.
 
 If ``PARENT_SCOPE`` is present then the variable is removed from the scope
 above the current scope.  See the same option in the :command:`set` command
 for further details.
 
+.. include:: UNSET_NOTE.txt
+
 Unset Environment Variable
 ^^^^^^^^^^^^^^^^^^^^^^^^^^