Add additional documentation for row filters.

Commit 52e4f0c added a feature to allow specifying row filters for
logical replication of tables. This patch adds detailed documentation on
that feature including examples to make it easier for users to understand.

Author: Peter Smith, Euler Taveira
Reviewed By: Greg Nancarrow, Aleksander Alekseev, Amit Kapila, Ajin Cherian, Alvaro Herrera
Discussion: https://postgr.es/m/CAHut+PtnsBr59=_NvxXp_=S-em0WxyuDOQmSTuHGb4sVhkHffg@mail.gmail.com

Author: Amit Kapila