[PATCH] add link to domain data types section from locale documentation

Hello.

While browsing character set collation documentation [1]
I've found myself a bit puzzled by unfamiliar terminology:
 > of course a domain over a collatable data type is collatable

Of course, it only takes a search or two to find out the exact
meaning of "domain" in this context, but I think it would be useful
to make this word a link to the relevant section as a help for less 
experienced users like myself.

I've attached a patch that does that. Please let me know whether this 
fix is desirable and correct (I'm new to Postgres) and if there is a 
better way of suggesting an improvement for the documentation.

[1] https://www.postgresql.org/docs/current/collation.html

-- 
Anton Voloshin
Postgres Professional: https://www.postgrespro.com
Russian Postgres Company