[PATCH 1 of 3] config: document the purpose of the `format` config section

[Boris Feld]

# HG changeset patch
# User Boris Feld <boris.feld at octobus.net>
# Date 1531987062 -7200
#      Thu Jul 19 09:57:42 2018 +0200
# Node ID 043803928d57813dd766747f9feea0d22c19b197
# Parent  88be288e8ac1c1125d896d2c25a515a664b87228
# EXP-Topic aggressivemergedeltas
# Available At https://bitbucket.org/octobus/mercurial-devel/
#              hg pull https://bitbucket.org/octobus/mercurial-devel/ -r 043803928d57
config: document the purpose of the `format` config section

The config section now have a top level documentation to clarify its intend and
usage. In particular, user are now explain when the option are taken in account
and how to convert repository.

There are an handful of experimental options in this section that does not match
its definition. They should be relocated to other section before getting out of
experimental. (see next changeset for one example).

diff --git a/mercurial/help/config.txt b/mercurial/help/config.txt
--- a/mercurial/help/config.txt
+++ b/mercurial/help/config.txt
@@ -790,6 +790,14 @@ Example for ``~/.hgrc``::
 ``format``
 ----------
 
+Configuration that control the repository format. Newer format option are more
+powerful but incompatible with some older version of Mercurial. Format options
+are considered at repository initialisation only. You need to make a new clone
+for config change to be taken in account.
+
+For more details about repository format and version compatibility see
+https://www.mercurial-scm.org/wiki/MissingRequirement
+
 ``usegeneraldelta``
     Enable or disable the "generaldelta" repository format which improves
     repository compression by allowing "revlog" to store delta against arbitrary