Change to_xfer_partial doc to use addressable memory units

Change to_xfer_partial doc to use addressable memory units

The commit

  d309493  target: consider addressable unit size when reading/writing memory

introduced the possibility of reading memory of targets with
non-8-bit-bytes (e.g. memories that store 16 bits at each address).
The documentation of target_read and target_write was updated
accordingly, but to_xfer_partial, which is very related, wasn't updated.
This commit fixes that.

gdb/ChangeLog:

	* target.h (struct target_ops) <to_xfer_partial>: Update doc to
	talk about addressable units instead of bytes.