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>