DOC: Improve documentation of the various hdr() fetches

DOC: Improve documentation of the various hdr() fetches

GitHub issue #796 notes that many administrators miss the fact that the `hdr()`
fetch (without the `f`) splits the header value at commas. This is only
mentioned at the end of a long paragraph.

This patch attempts to improve the documentation by:
- Explaning the "comma issue" as early as possible.
- Adding newlines to split the explanation into distinct sections.
- Reducing duplication by making the `res` siblings refer to their `req`
  counterparts.

This patch may be backported as long as it applies cleanly. During the
refactoring I needed to adjust several explanations for consistency and not all
of them might be available in older branches.

(cherry picked from commit 27c70ae23c1f5b39c9757387559072a096c8aa8c)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>