Doc enhancements

[Thom Brown <thombrown(at)gmail(dot)com>]

Hi,

I was wondering if I could propose having jump-links at the top of the Index
page of the PostgreSQL documentation?  Like A - B - C - D etc which would
jump to that point in the index.

Also I noticed the keyword PERFORM wasn't listed in the index.  It might be
useful to list it, and also to mention it on the SELECT keyword page.

Thanks

Thom