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

1 files changed, 5 insertions, 0 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