Documentation/describe: improve one-line summary

git describe does not show 'the most recent tag that is reachable from a
commit', but a descriptive name based on this tag. Fix the description to
reflect that.

Suggested-by: Albert Netymk <albertnetymk@gmail.com>
Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-describe.txt

@@ -3,7 +3,7 @@ git-describe(1)
 
 NAME
 ----
-git-describe - Show the most recent tag that is reachable from a commit
+git-describe - Describe a commit using the most recent tag reachable from it
 
 
 SYNOPSIS