format-patch (doc): clarify --base=auto

What --base=auto tells format-patch is to compute the base commit
itself, using the tracking information.  It does not make anything
track anything.

Tighten the phrasing so that it won't be copied and pasted to other
places.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-format-patch.txt

@@ -689,10 +689,10 @@ You can also use `git format-patch --base=P -3 C` to generate patches
 for A, B and C, and the identifiers for P, X, Y, Z are appended at the
 end of the first message.
 
-If set `--base=auto` in cmdline, it will track base commit automatically,
-the base commit will be the merge base of tip commit of the remote-tracking
+If set `--base=auto` in cmdline, it will automatically compute
+the base commit as the merge base of tip commit of the remote-tracking
 branch and revision-range specified in cmdline.
-For a local branch, you need to track a remote branch by `git branch
+For a local branch, you need to make it to track a remote branch by `git branch
 --set-upstream-to` before using this option.
 
 EXAMPLES