merge: document checkignored and checkunknown configs again

These options were undocumented for 3.7 because of an issue found during the
freeze (see rev 7cb7264cfd52). This issue has now been fixed, so we can
document these options again.

mercurial/help/config.txt

@@ -1006,6 +1006,25 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``
 ------------------