Doc: update src/tools/pgindent/README for current practice.

Doc: update src/tools/pgindent/README for current practice.

This README explains how to run pgindent, but it was written
for our former practice of running pgindent only infrequently.
Nowadays the plan is to keep the tree indent-clean all the time,
so the typical thing is to do an incremental pgindent run with
each commit.  Revise to explain that as the normal case, and
the infrequent full-on process as a separate thing.

For now, pgperltidy is still a run-it-infrequently item.