pgsql: Update assorted TOAST-related documentation.

Update assorted TOAST-related documentation.

While working on documentation for expanded arrays, I noticed a number of
details in the TOAST-related documentation that were already inaccurate or
obsolete. This should be fixed independently of whether expanded arrays
get in or not. One issue is that the already existing indirect-pointer
facility was not documented at all. Also, the documentation says that you
only need to use VARSIZE/SET_VARSIZE if you've made your variable-length
type TOAST-aware, but actually we've forced that business on all varlena
types even if they've opted out of TOAST by setting storage = plain.
Wordsmith a few other things too, like an amusingly archaic claim that
there are few 64-bit machines.

I thought about back-patching this, but since all this doco is oriented
to hackers and C-coded extension authors, fixing it in HEAD is probably
good enough.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/9bb955c8286c20474b5462eea3e3cf76c694d88f

Modified Files
--------------
doc/src/sgml/ref/create_type.sgml | 25 +++++--
doc/src/sgml/storage.sgml | 143 +++++++++++++++++++++++++++----------
doc/src/sgml/xtypes.sgml | 52 ++++++++------
3 files changed, 157 insertions(+), 63 deletions(-)