[PATCH] More docs on what to do and not do in extension code

Hi folks

The attached patch expands the xfunc docs and bgworker docs a little,
providing a starting point for developers to learn how to do some common
tasks the Postgres Way.

It mentions in brief these topics:

* longjmp() based exception handling with elog(ERROR), PG_CATCH() and
PG_RE_THROW() etc
* Latches, spinlocks, LWLocks, heavyweight locks, condition variables
* shm, DSM, DSA, shm_mq
* syscache, relcache, relation_open(), invalidations
* deferred signal handling, CHECK_FOR_INTERRUPTS()
* Resource cleanup hooks and callbacks like on_exit, before_shmem_exit, the
resowner callbacks, etc
* signal handling in bgworkers

All very superficial, but all things I really wish I'd known a little
about, or even that I needed to learn about, when I started working on
postgres.

I'm not sure it's in quite the right place. I wonder if there should be a
separate part of xfunc.sgml that covers the slightly more advanced bits of
postgres backend and function coding like this, lists relevant README files
in the source tree, etc.

I avoided going into details like how resource owners work. I don't want
the docs to have to cover all that in detail; what I hope to do is start
providing people with clear references to the right place in the code,
READMEs, etc to look when they need to understand specific topics.