[PATCH] Add section headings to index types doc

Hi hackers,

Every time I have to look up what kinds of operations each index type is
suitable for, I get annoyed by the index types page being virtually
unskimmable due to not having headings for each index type.

Attached is a patch that adds <sect2> tags for each index type to make
it easier to see where the description of each one starts.

- ilmari
-- 
"The surreality of the universe tends towards a maximum" -- Skud's Law
"Never formulate a law or axiom that you're not prepared to live with
 the consequences of."                              -- Skud's Meta-Law