doc: Improve documentation about MI thread output

doc: Improve documentation about MI thread output

I noticed that the documentation on how the info about threads is output
in MI is duplicated and not up to date.  The duplication is between the
"GDB/MI Thread Information" page and the -thread-info result
description.

I improved the "GDB/MI Thread Information" page a bit and referred to it
in the -thread-info doc.  This way, the -thread-info doc is more precise
(it did not mention the "threads" and "current-thread-id" attributes)
and concise.

gdb/doc/ChangeLog:

	* gdb.texinfo (GDB/MI Thread Information): Add missing
	fields, re-word some things.
	(GDB/MI Thread Commands): Describe fields found in the output of
	-thread-info, remove description of fields in the
	thread output tuple, replace with a cross-reference to "GDB/MI
	Thread Information".