doc:git-for-each-ref: fix styling and typos

This commit fixes the synopsis syntax and changes the wording of a few
descriptions to be more consistent with the rest of the documentation.

It is a prepartion for the next commit that checks that synopsis style is
applied consistently across a manual page.

Signed-off-by: Jean-Noël Avila <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: jnavila