Change version scheme to use only two components for feature levels

Historically CMake used three version components for the feature level.
We released new features while incrementing only the third version
component.  Since commit v2.8.2~105^2~4 (New version scheme to support
branchy workflow, 2010-04-23) we used the fourth version component for
bug-fix releases and the development date:

 <major>.<minor>.<patch>[.<tweak>][-rc<n>] = Release
 <major>.<minor>.<patch>.<date>[-<id>]     = Development

This solidified use of three components for the feature level, and was
necessary to continue releasing 2.x versions because:

* Some existing projects performed floating-point comparisons of
  ${CMAKE_MAJOR_VERSION}.${CMAKE_MINOR_VERSION} to 2.x numbers
  so ``x`` could never be higher than 9.

* Version 2.9.<date> was used briefly in post-2.8.0 development in
  CVS prior to the transition to Git, so using it in releases may
  have caused confusion.

Now that we are moving to 3.x versions, these two restrictions go away.
Therefore we now change to use only two components for the feature
level and use the scheme:

 <major>.<minor>.<patch>[-rc<n>] = Release
 <major>.<minor>.<date>[-<id>]   = Development

3 files changed, 25 insertions, 9 deletions

diff --git a/Help/release/3.0.0.rst b/Help/release/3.0.0.rst
index 105d651..208e92a 100644
--- a/Help/release/3.0.0.rst
+++ b/Help/release/3.0.0.rst
@@ -390,6 +390,11 @@ Deprecated and Removed Features
 Other Changes
 =============
 
+* The version scheme was changed to use only two components for
+  the feature level instead of three.  The third component will
+  now be used for bug-fix releases or the date of development versions.
+  See the :variable:`CMAKE_VERSION` variable documentation for details.
+
 * The default install locations of CMake itself on Windows and
   OS X no longer contain the CMake version number.  This allows
   for easy replacement without re-generating local build trees
diff --git a/Help/variable/CMAKE_TWEAK_VERSION.rst b/Help/variable/CMAKE_TWEAK_VERSION.rst
index a2c8f35..be2e050 100644
--- a/Help/variable/CMAKE_TWEAK_VERSION.rst
+++ b/Help/variable/CMAKE_TWEAK_VERSION.rst
@@ -1,5 +1,11 @@
 CMAKE_TWEAK_VERSION
 -------------------
 
-Fourth version number component of the :variable:`CMAKE_VERSION`
-variable.
+Defined to ``0`` for compatibility with code written for older
+CMake versions that may have defined higher values.
+
+.. note::
+
+  In CMake versions 2.8.2 through 2.8.12, this variable holds
+  the fourth version number component of the
+  :variable:`CMAKE_VERSION` variable.
diff --git a/Help/variable/CMAKE_VERSION.rst b/Help/variable/CMAKE_VERSION.rst
index 6184f08..bbb1d91 100644
--- a/Help/variable/CMAKE_VERSION.rst
+++ b/Help/variable/CMAKE_VERSION.rst
@@ -1,24 +1,23 @@
 CMAKE_VERSION
 -------------
 
-The CMake version string as up to four non-negative integer components
+The CMake version string as three non-negative integer components
 separated by ``.`` and possibly followed by ``-`` and other information.
-The first three components represent the feature level and the fourth
+The first two components represent the feature level and the third
 component represents either a bug-fix level or development date.
 
 Release versions and release candidate versions of CMake use the format::
 
-  <major>.<minor>.<patch>[.<tweak>][-rc<n>]
+  <major>.<minor>.<patch>[-rc<n>]
 
-where the ``<tweak>`` component is less than ``20000000``.  Development
+where the ``<patch>`` component is less than ``20000000``.  Development
 versions of CMake use the format::
 
-  <major>.<minor>.<patch>.<date>[-<id>]
+  <major>.<minor>.<date>[-<id>]
 
 where the ``<date>`` component is of format ``CCYYMMDD`` and ``<id>``
 may contain arbitrary text.  This represents development as of a
-particular date following the ``<major>.<minor>.<patch>`` feature
-release.
+particular date following the ``<major>.<minor>`` feature release.
 
 Individual component values are also available in variables:
 
@@ -35,6 +34,12 @@ strings as floating-point numbers.
 
 .. note::
 
+  CMake versions 2.8.2 through 2.8.12 used three components for the
+  feature level.  Release versions represented the bug-fix level in a
+  fourth component, i.e. ``<major>.<minor>.<patch>[.<tweak>][-rc<n>]``.
+  Development versions represented the development date in the fourth
+  component, i.e. ``<major>.<minor>.<patch>.<date>[-<id>]``.
+
   CMake versions prior to 2.8.2 used three components for the
   feature level and had no bug-fix component.  Release versions
   used an even-valued second component, i.e.