Documentation/git-worktree: associate options with commands

git-worktree options affect some worktree commands but not others, but
this is not necessarily obvious from the option descriptions. Make this
clear by indicating explicitly which commands are affected by which
options.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-worktree.txt

@@ -28,15 +28,15 @@ OPTIONS
 
 -n::
 --dry-run::
-	Do not remove anything; just report what it would
+	With `prune`, do not remove anything; just report what it would
 	remove.
 
 -v::
 --verbose::
-	Report all removals.
+	With `prune`, report all removals.
 
 --expire <time>::
-	Only expire unused worktrees older than <time>.
+	With `prune`, only expire unused worktrees older than <time>.
 
 SEE ALSO
 --------