A few updates to the confd design (2.1)

While the 2.1 design is old and should be “immutable”, I can't find
documentation about the confd protocol anywhere else, so let's correct
the design doc.

The patch is mostly style changes, plus a clarification on the ‘query’
field of the request, which varies *a lot* per request type.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>