persistent-nodemap: document the `devel.persistent-nodemap` option

What it does is not obvious.

Differential Revision: https://phab.mercurial-scm.org/D9753

mercurial/configitems.py

@@ -635,6 +635,8 b' coreconfigitem('
     b'legacy.exchange',
     default=list,
 )
+# When True, revlogs use a special reference version of the nodemap, that is not
+# performant but is "known" to behave properly.
 coreconfigitem(
     b'devel',
     b'persistent-nodemap',