doc: convert git-apply synopsis and options to new style

doc: convert git-apply synopsis and options to new style

Convert git-apply from [verse]/single-quote style to the modern
synopsis-block style:

- Replace [verse] with [synopsis] in SYNOPSIS block
- Backtick-quote all OPTIONS terms and config keys in config/apply.adoc
- Convert single-quoted inline commands ('git apply', 'diff', etc.)
- Wrap standalone placeholders in underscores (<n>, <root>, <action>)
- Backtick-quote `*.rej` and GNU `patch` tool references

Signed-off-by: Jean-Noël Avila <jn.avila@free.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>