NEWS/manual: Missing documentation for new commands in 7.6.

NEWS/manual: Missing documentation for new commands in 7.6.

I hacked "apropos" to dump the whole set of commands (just make it
accept the entry string as regex), and then diffed the output of 7.5
vs 7.6, --enable-targets=all builds.  That allowed then checking
whether some commands had not been mentioned in NEWS or the manual.
These are what I found missing.  We've been a bit negligent in
requiring documentation bits for debug commands.

gdb/
2013-04-02  Pedro Alves  <palves@redhat.com>

	* NEWS: Mention "set/show debug aarch64", "set/show debug
	coff-pe-read" and "set/show debug mach-o".

gdb/doc/
2013-04-02  Pedro Alves  <palves@redhat.com>

	* gdb.texinfo (Debugging Output): Document "set/show debug
	aarch64", "set/show debug coff-pe-read" and "set/show debug
	mach-o".