doc: rewrite description for rev-parse --short

`git rev-parse --short` is not a generic modifier but just a variant
of `--verify` and considers the given length only as a suggestion to
ensure uniqueness.

Signed-off-by: Andreas Heiduk <asheiduk@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -862,6 +862,7 @@ core.abbrev::
 	computed based on the approximate number of packed objects
 	in your repository, which hopefully is enough for
 	abbreviated object names to stay unique for some time.
+	The minimum length is 4.
 
 add.ignoreErrors::
 add.ignore-errors (deprecated)::

Documentation/git-rev-parse.txt

@@ -126,6 +126,12 @@ can be used.
 	'git diff-{asterisk}'). In contrast to the `--sq-quote` option,
 	the command input is still interpreted as usual.
 
+--short[=length]::
+	Same as `--verify` but shortens the object name to a unique
+	prefix with at least `length` characters. The minimum length
+	is 4, the default is the effective value of the `core.abbrev`
+	configuration variable (see linkgit:git-config[1]).
+
 --not::
 	When showing object names, prefix them with '{caret}' and
 	strip '{caret}' prefix from the object names that already have
@@ -136,12 +142,6 @@ can be used.
 	The option core.warnAmbiguousRefs is used to select the strict
 	abbreviation mode.
 
---short::
---short=number::
-	Instead of outputting the full SHA-1 values of object names try to
-	abbreviate them to a shorter unique name. When no length is specified
-	7 is used. The minimum length is 4.
-
 --symbolic::
 	Usually the object names are output in SHA-1 form (with
 	possible '{caret}' prefix); this option makes them output in a