* gdb.texinfo (Packets): Move information out of 'q' and 'Q'

* gdb.texinfo (Packets): Move information out of 'q' and 'Q'
entries into the General Query Packets section.  Add a
cross-reference to that section.  Drop description of replies, as
these are covered in the descriptions of each packet.
(General Query Packets): Add introductory text.  Explain naming
conventions, and how the end of a name is recognized.