Change :for-macro from set to a single symbol, and add :reason key

:reason value is a string containing an explanation of why the
suppression condition was created in the first place.
Add :reason for disable-warning config that I forgot to write one for
First version of some code to enable fine-grained disabling of warnings

Only for one kind of :suspicious-expression warning so far, involving
macros.  Will extend to suppressing other types of warnings over time.
I may come up with a general way of doing this for all or most types
of warnings, but I would rather generalize from several examples
rather than only one.