pgsql: Remove option to change parser of an existing text search

Log Message:
-----------
Remove option to change parser of an existing text search configuration.
This prevents needing to do complex and poorly-defined updates of the
mapping table if the new parser has different token types than the old.
Per discussion.

Modified Files:
--------------
pgsql/doc/src/sgml/ref:
alter_tsconfig.sgml (r1.1 -> r1.2)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/alter_tsconfig.sgml?r1=1.1&r2=1.2)
create_tsconfig.sgml (r1.1 -> r1.2)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/create_tsconfig.sgml?r1=1.1&r2=1.2)
pgsql/src/backend/commands:
tsearchcmds.c (r1.3 -> r1.4)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/tsearchcmds.c?r1=1.3&r2=1.4)
pgsql/src/backend/parser:
gram.y (r2.599 -> r2.600)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/gram.y?r1=2.599&r2=2.600)
pgsql/src/include/nodes:
parsenodes.h (r1.351 -> r1.352)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/nodes/parsenodes.h?r1=1.351&r2=1.352)