Documentation: explain the meaning of "-g" in git-describe output

Signed-off-by: Markus Heidelberg <markus.heidelberg@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-describe.txt

@@ -105,6 +105,9 @@ The number of additional commits is the number
 of commits which would be displayed by "git log v1.0.4..parent".
 The hash suffix is "-g" + 7-char abbreviation for the tip commit
 of parent (which was `2414721b194453f058079d897d13c4e377f92dc6`).
+The "g" prefix stands for "git" and is used to allow describing the version of
+a software depending on the SCM the software is managed with. This is useful
+in an environment where people may use different SCMs.
 
 Doing a 'git describe' on a tag-name will just show the tag name: