pgsql: Update documentation to reflect that standard PLs are now extens

Update documentation to reflect that standard PLs are now extensions.

Recommend use of CREATE EXTENSION rather than plain CREATE LANGUAGE
where relevant.  Encourage PL authors to provide extension wrappers
for their PLs.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/c2903fb3d2aeb0a7206945478ad28bc47bd6c6e8

Modified Files
--------------
doc/src/sgml/installation.sgml        |    4 ++--
doc/src/sgml/plhandler.sgml           |    9 +++++++++
doc/src/sgml/plperl.sgml              |   10 +++++-----
doc/src/sgml/plpython.sgml            |    1 +
doc/src/sgml/pltcl.sgml               |    1 +
doc/src/sgml/ref/create_language.sgml |   15 +++++++++++++++
doc/src/sgml/ref/drop_language.sgml   |    9 +++++++++
doc/src/sgml/xplang.sgml              |   21 ++++++++++-----------
8 files changed, 52 insertions(+), 18 deletions(-)