pgsql: Update comments for PG_DETOAST_PACKED and VARDATA_ANY on a

Log Message:
-----------
Update comments for PG_DETOAST_PACKED and VARDATA_ANY on a structures
that require alignment.

Add a paragraph to the "User-Defined Types" chapter on using these
macros since it seems like they're a hit.

Gregory Stark

Modified Files:
--------------
pgsql/doc/src/sgml:
xtypes.sgml (r1.28 -> r1.29)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/xtypes.sgml.diff?r1=1.28&r2=1.29)
pgsql/src/include:
fmgr.h (r1.50 -> r1.51)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/fmgr.h.diff?r1=1.50&r2=1.51)
postgres.h (r1.80 -> r1.81)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/postgres.h.diff?r1=1.80&r2=1.81)