doc: clarify command equivalence comment

Documentation of command parsing for :(optional) includes a terse
comment; expand it to be clearer to readers.

Suggested-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: D. Ben Knoble <ben.knoble+github@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: benknoble

Documentation/gitcli.adoc

@@ -223,7 +223,7 @@ Options that take a filename allow a prefix `:(optional)`. For example:
 
 ----------------------------
 git commit -F :(optional)COMMIT_EDITMSG
-# if COMMIT_EDITMSG does not exist, equivalent to
+# if COMMIT_EDITMSG does not exist, the above is equivalent to
 git commit
 ----------------------------