Some minor improvements to logical decoding document.

Also improve help message in pg_recvlogical.
Fix memory leak during regular expression execution.

For a regex containing backrefs, pg_regexec() might fail to free all the
sub-DFAs that were created during execution, resulting in a permanent
(session lifespan) memory leak.  Problem was introduced by me in commit
587359479acbbdc95c8e37da40707e37097423f5.  Per report from Sandro Santilli;
diagnosis by Greg Stark.
Setup error context callback for transaction lock waits

With this in place, a session blocking behind another one because of
tuple locks will get a context line mentioning the relation name, tuple
TID, and operation being done on tuple.  For example:

LOG:  process 11367 still waiting for ShareLock on transaction 717 after 1000.108 ms
DETAIL:  Process holding the lock: 11366. Wait queue: 11367.
CONTEXT:  while updating tuple (0,2) in relation "foo"
STATEMENT:  UPDATE foo SET value = 3;

Most usefully, the new line is displayed by log entries due to
log_lock_waits, although of course it will be printed by any other log
message as well.

Author: Christian Kruse, some tweaks by Álvaro Herrera
Reviewed-by: Amit Kapila, Andres Freund, Tom Lane, Robert Haas