i965: Add a pile of comments to brw_queryobj.c.

i965: Add a pile of comments to brw_queryobj.c.

This code was really difficult to follow, for a number of reasons:

- Queries were handled in four different ways (TIMESTAMP writes a single
  value, TIME_ELAPSED writes a single pair of values, occlusion queries
  write pairs of values for the start and end of each batch, and other
  queries are done entirely in software.  It turns out that there are
  very good reasons each query is handled the way it is, but
  insufficient comments explaining the rationale.

- It wasn't immediately obvious which functions were driver hooks
  and which were helper functions.  For example, brw_query_begin() is
  a driver hook that implements glBeginQuery() for all query types, but
  the similarly named brw_emit_query_begin() is a helper function that's
  only relevant for occlusion queries.

Extra explanatory comments should save me and others from constantly
having to ask how this code works and why various query types are
handled differently.

v2: Incorporate Eric's feedback: change "as soon as possible" to "the
    results will be present when mapped."

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>