merge: undocument checkunknown and checkignored configs for 3.7

We've discovered an issue with this flag during certain kinds of rebases. When:

(1) we're rebasing while currently on the destination commit, and
(2) an untracked or ignored file F is currently in the working copy, and
(3) the same file F is in a source commit, and
(4) F has different contents in the source commit,

then we'll try to merge the file rather than overwrite it.

An earlier patch I sent honored the options for these situations as well.
Unfortunately, rebases go through the same flow as the old, deprecated 'hg
merge --force'. We'd rather not make any changes to 'hg merge --force'
behavior, and there's no way from this point in the code to figure out whether
we're in 'hg rebase' or 'hg merge --force'.

Pierre-Yves David and I came up with the idea to split the 'force' flag up into
'force' for rebases, and 'forcemerge' for merge. Since this is a very
disruptive change and we're in freeze mode, simply undocument the options for
this release so that our hands aren't tied by BC concerns. We'll redocument
them in the next release.

mercurial/help/config.txt

@@ -1006,25 +1006,6 b' proxy.'
     Optional. Always use the proxy, even for localhost and any entries
     in ``http_proxy.no``. (default: False)
 
-``merge``
----------
-
-This section specifies behavior during merges and updates.
-
-``checkignored``
-   Controls behavior when an ignored file on disk has the same name as a tracked
-   file in the changeset being merged or updated to, and has different
-   contents. Options are ``abort``, ``warn`` and ``ignore``. With ``abort``,
-   abort on such files. With ``warn``, warn on such files and back them up as
-   .orig. With ``ignore``, don't print a warning and back them up as
-   .orig. (default: ``abort``)
-
-``checkunknown``
-   Controls behavior when an unknown file that isn't ignored has the same name
-   as a tracked file in the changeset being merged or updated to, and has
-   different contents. Similar to ``merge.checkignored``, except for files that
-   are not ignored. (default: ``abort``)
-
 ``merge-patterns``
 ------------------