Clarify doc about memory read/write and non-8-bits addressable memory unit sizes

Clarify doc about memory read/write and non-8-bits addressable memory unit sizes

New in v3:

 * Change RSP documentation as well. The m, M and X packets now use
 lengths in addressable memory units.

New in v2:

 * Change wording: use byte for 8-bits chunks and addressable memory unit
   for the unit of data associated to a single address.
 * Introduce definition of addressable memory unit in the Memory
   section.

This patch modifies the manual to clarify the MI, RSP and Python APIs in
regard to reading/writing memory on architectures with addressable
memory unit that are not 8 bits.

Care is taken to use the word "addressable memory unit" or "memory unit"
when referring to one piece of the smallest addressable size on the
current architecture and the word "byte" when referring to an 8-bits
data piece.

For MI, -data-{read,write}-memory are not modified, since they are
deprecated.

gdb/doc/ChangeLog:

	* gdb.texinfo (GDB/MI Data Manipulation): Clarify usage of
	bytes and memory units for -data-{read,write}-memory-bytes.
	(Packets): Same for m, M and X packets.
	* python.texi (Inferiors In Python): Same for read_memory and
	write_memory.