Clarify the inspect.getmembers predicate parameter (#6615)

Previously, the predicate parameter was mentioned, but what it was to be
called with was not documented and required either trial-and-error or
looking into the source to find that it is called with the `value`, or
second item, of the full members list. This change addresses what the
predicate will receive, as well as does some light formatting to make
this clear.