Documentation/pretty-{formats,options}: better reference for "format:<string>"

Documentation/pretty-{formats,options}: better reference for "format:<string>"

In "git help log" (and friends) it's not easy to find the possible
placeholder for <string> for the "--pretty=format:<string>" option
to git log.

This patch makes the placeholder easier to find by adding a reference
to the "PRETTY FORMATS" section and repeating the "format:<string>"
phrase.

Signed-off-by: Nazri Ramliy <ayiehere@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>