make libpq documentation navigable between functions

Hello dev,

While writing some libpq code, I found it quite irritating that the
documentation is not navigable, so when a function appears in a
description of another function and you are interested, there is no direct
way to find it, you have to go to the index or to guess in which section
it is going to appear.

Attached:
- a first patch to add a few missing "id"
- a script which adds the references
- a second patch which is the result of applying the script
on top of the first patch, so that all PQ* functions are
replaced by links to their documentation.

While doing this, I noticed that two functions are not documented:
PQregisterThreadLock and PQsetResultInstanceData.

--
Fabien.