tag doc: reword --[no-]merged to talk about commits, not tips

Change the wording for the --merged and --no-merged options to talk
about "commits" instead of "tips".

This phrasing was copied from the "branch" documentation in commit
5242860f54 ("tag.c: implement '--merged' and '--no-merged' options",
2015-09-10). Talking about the "tip" is branch nomenclature, not
something usually associated with tags.

This phrasing might lead the reader to believe that these options
might find tags pointing to trees or blobs, let's instead be explicit
and only talk about commits.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-tag.txt

@@ -125,11 +125,11 @@ This option is only applicable when listing tags without annotation lines.
 	specified).
 
 --merged [<commit>]::
-	Only list tags whose tips are reachable from the specified commit
-	(`HEAD` if not specified).
+	Only list tags whose commits are reachable from the specified
+	commit (`HEAD` if not specified).
 
 --no-merged [<commit>]::
-	Only list tags whose tips are not reachable from the specified
+	Only list tags whose commits are not reachable from the specified
 	commit (`HEAD` if not specified).
 
 --points-at <object>::