for-each-ref: fix documentation argument ordering

for-each-ref: fix documentation argument ordering

Improve the 'git-for-each-ref(1)' documentation with two corrections:

1. Add parentheses around `--exclude=<pattern>` to indicate this option
   can be repeated as a complete unit.

2. Move `--stdin | <pattern> ...` to the end, after all flags, since
   `<pattern>` is a positional argument that should appear last in the
   argument list.

While here, change to using the synopsis block which will automatically
format placeholders in italics and keywords in monospace.

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>