doc: add more pointers to gitattributes(5) for userdiff

doc: add more pointers to gitattributes(5) for userdiff

Several Git commands can make use of the builtin userdiff patterns, but
it's not obvious in the documentation. Add pointers to the 'Defining a
custom hunk header' part of gitattributes(5) in the description of the
following options:

- the '--function-context' option of `git diff` and friends
- the '--function-context' option of `git grep`
- the '-L :<funcname>' option of `git log`, `gitk` and `git blame`

In 'git-grep.txt', take the opportunity to use backticks in the
description of '--show-function', and improve the wording of the
desription of '--function-context'.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>