gdb/doc: Restructure frame command documentation.

gdb/doc: Restructure frame command documentation.

The 'frame' command is documented in two places.  The 'select-frame'
command is only mentioned in one of these places.

Of the two places, having the description of 'frame' and 'select-frame'
in the section 'Selecting a Frame' seems like the most obvious choice,
which is where things like 'up' and 'down' are also documented.

This commit moves the documentation of 'frame' and 'select-frame' into
the 'Selecting a Frame' section, and removes the duplicate documentation
of the 'frame' command.

At the same time I have reordered the sections in the 'Examining the
Stack' chapter, moving the discussion of frame filters to the end of the
chapter; it feels odd that we talk about frame filters before such basic
things like navigating the stack, or examining stack frames in general.

gdb/doc/ChangeLog:

	* gdb.texinfo (Frames): Remove 'frame' and 'select-frame'
	description.
	(Frame Filter Management): Move to later in the 'Examining the
	Stack' chapter.
	(Selection): Add entry for 'select-frame'.