Policy documentation improvements

In ALTER POLICY, use 'check_expression' instead of 'expression' for the
parameter, to match up with the recent CREATE POLICY change.

In CREATE POLICY, frame the discussion as granting access to rows
instead of limiting access to rows.  Further, clarify that the
expression must return true for rows to be visible/allowed and that a
false or NULL result will mean the row is not visible/allowed.

Per discussion with Dean Rasheed and Robert.