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.

gdb/doc/python.texi

@@ -2749,7 +2749,7 @@ return an empty tuple.
 
 @findex Inferior.read_memory
 @defun Inferior.read_memory (address, length)
-Read @var{length} bytes of memory from the inferior, starting at
+Read @var{length} addressable memory units from the inferior, starting at
 @var{address}.  Returns a buffer object, which behaves much like an array
 or a string.  It can be modified and given to the
 @code{Inferior.write_memory} function.  In @code{Python} 3, the return
@@ -2762,7 +2762,8 @@ Write the contents of @var{buffer} to the inferior, starting at
 @var{address}.  The @var{buffer} parameter must be a Python object
 which supports the buffer protocol, i.e., a string, an array or the
 object returned from @code{Inferior.read_memory}.  If given, @var{length}
-determines the number of bytes from @var{buffer} to be written.
+determines the number of addressable memory units from @var{buffer} to be
+written.
 @end defun
 
 @findex gdb.search_memory