Update Python unwinder documentation

Update Python unwinder documentation

PR python/19808 points out a few issues in the Python unwinder
documentation.  This patch update the documentation for
create_unwind_info and read_register to address the issues noted, and
adds a cautionary note about writing an unwinder.

gdb/doc/ChangeLog
2018-09-10  Tom Tromey  <tom@tromey.com>

	PR python/19808:
	* python.texi (Unwinding Frames in Python): Rewrite
	create_unwind_info documentation.  Update read_register
	documentation and add a note about unwinder caution.