censor: document that some commands simply ignore censored data

I can't see a benefit for hg grep to ever error out early when it
encounters censored data.

hgext/censor.py

@@ -23,6 +23,9 b' simply fail when asked to produce censor'
 ``hg update``, must be capable of tolerating censored data to continue to
 function in a meaningful way. Such commands only tolerate censored file
 revisions if they are allowed by the "censor.policy=ignore" config option.
+
+A few informative commands such as ``hg grep`` will unconditionally
+ignore censored data and merely report that it was encountered.
 """
 
 from __future__ import absolute_import