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).

mercurial/help/config.txt

@@ -790,6 +790,14 b' Example for ``~/.hgrc``::'
 ``format``
 ----------
 
+Configuration that controls the repository format. Newer format options are more
+powerful but incompatible with some older versions of Mercurial. Format options
+are considered at repository initialization only. You need to make a new clone
+for config change to be taken into 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