Clarify the deprecation of platform.dist().

Add versionadded tags.

Doc/library/platform.rst

@@ -234,7 +234,15 @@ Unix Platforms
 
 .. function:: dist(distname='', version='', id='', supported_dists=('SuSE','debian','redhat','mandrake',...))
 
-   This is another name for :func:`linux_distribution`.
+   This is an old version of the functionality now provided by
+   :func:`linux_distribution`. For new code, please use the
+   :func:`linux_distribution`.
+
+   The only difference between the two is that ``dist()`` always
+   returns the short name of the distribution taken from the
+   ``supported_dists`` parameter.
+
+   .. deprecated:: 2.6
 
 .. function:: linux_distribution(distname='', version='', id='', supported_dists=('SuSE','debian','redhat','mandrake',...), full_distribution_name=1)
 
@@ -252,6 +260,8 @@ Unix Platforms
    parameters.  ``id`` is the item in parentheses after the version number.  It
    is usually the version codename.
 
+   .. versionadded:: 2.6
+
 .. function:: libc_ver(executable=sys.executable, lib='', version='', chunksize=2048)
 
    Tries to determine the libc version against which the file executable (defaults