discovery: document the `devel.discovery.randomize` option

Gratuitous improvement as I was passing by this config section to add more.

Differential Revision: https://phab.mercurial-scm.org/D9797

mercurial/configitems.py

@@ -700,6 +700,8 b' coreconfigitem('
     b'debug.peer-request',
     default=False,
 )
+# If discovery.randomize is False, random sampling during discovery are
+# deterministic. It is meant for integration tests.
 coreconfigitem(
     b'devel',
     b'discovery.randomize',