doc/config: do a better job of introducing 'worktree.guessRemote'

The documentation for this option jumps right in with "With `add`",
without explaining that `add` is a sub-command of "git worktree".
Together with rather odd grammatical structure of the remainder of the
sentence, the description can be difficult for newcomers to understand.
Clarify by improving the grammar and mentioning "git worktree add"
explicitly.

Reported-by: Олег Самойлов <splarv@ya.ru>
Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config/worktree.txt

@@ -1,6 +1,6 @@
 worktree.guessRemote::
-	With `add`, if no branch argument, and neither of `-b` nor
+	If no branch is specified and neither `-b` nor `-B` nor
-	`-B` nor `--detach` are given, the command defaults to
+	`--detach` is used, then `git worktree add` defaults to
 	creating a new branch from HEAD.  If `worktree.guessRemote` is
 	set to true, `worktree add` tries to find a remote-tracking
 	branch whose name uniquely matches the new branch name.  If