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)