Add <caution> for ALTER TEXT SEARCH CONFIGURATION

If you alter one of the built-in text search configurations, the
modifications to it will not get dumped by pg_dump, and thus won't get
propagated by pg_upgrade leading to silent behavior changes in the new
cluster (as well in any other type of restoration from pg_dump output)

I would say it is bad practise to alter one of those anyway, rather than
copy and alter the copy. But I wasn't aware until now of just how bad of
an idea it was. I think that this needs to be warned about in the docs
for ALTER TEXT SEARCH CONFIGURATION. If I were monkeying around in
$SHAREDLIB/tsearch_data, I'd expect traps like this, but not when executing
things from SQL after reading the docs on the command.

Cheers,

Jeff