[DOCS] Add link to current_schemas in config.sgml

Hi folks, This is just a quick docs patch to add a link to the mention of the current_schemas function from 18.10.1. Statement Behavior. The function must be called with one boolean argument, the meaning of which isn't clear until you read the description in 9.23. I also removed the parentheses

[DOCS] DOCS: SGML identifier may not exceed 44 characters

Hi folks, I was working on a little docs patch today, and when I tried to `make`, openjade choked on an identifier in information_schema.sgml, which is very much unrelated to my changes: openjade:information_schema.sgml:828:60:Q: length of name token must not exceed NAMELEN (44) Here is a trivia