[binutils-gdb] Change to_xfer_partial doc to use addressable memory units

[sergiodj+buildbot@sergiodj.net]

*** TEST RESULTS FOR COMMIT e4da2c61669d199c75b96a1be25e13e3b8254e89 ***

Author: Simon Marchi <simon.marchi@ericsson.com>
Branch: master
Commit: e4da2c61669d199c75b96a1be25e13e3b8254e89

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.