Improve RLS documentation with respect to COPY

Improve RLS documentation with respect to COPY

Documentation for pg_restore said COPY TO does not support row security
when in fact it should say COPY FROM. Fix that.

While at it, make it clear that "COPY FROM" does not allow RLS to be
enabled and INSERT should be used instead. Also that SELECT policies
will apply to COPY TO statements.

Back-patch to 9.5 where RLS first appeared.

Author: Joe Conway
Reviewed-By: Dean Rasheed and Robert Haas
Discussion: https://postgr.es/m/5744FA24.3030008%40joeconway.com