doc: update openocd.texi after change of gdb-attach default value

doc: update openocd.texi after change of gdb-attach default value

While on it
- change some occurrences of gdb to GDB if it refers GDB software
  (as oposed to a gdb command)
- add some xrefs
- give more meaningful example of target event definition
- remove obsoleted example of GDB hook-step

Change-Id: Ia2e26021d57f675acfa1de704f6c3e81c22bb8bf
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/4444
Tested-by: jenkins
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>