Document operate-and-get-next

Document operate-and-get-next

This adds some documentation for the operate-and-get-next readline
function that gdb supplies.  The text is largely taken from the Bash
manual.

gdb/doc/ChangeLog
2019-11-11  Tom Tromey  <tom@tromey.com>

	* gdb.texinfo (Editing): Document operate-and-get-next.

Change-Id: I9adb16d9ce84bfbda5fe8a2828f668ea878c080c