doc/openocd.texi: Document find and ocd_find command

doc/openocd.texi: Document find and ocd_find command

Document both find and ocd_find command under Config File Guidelines
-> Interface Config Files.  find command is used in the previous
section as well but the previous section is more about using OpenOCD.

The section added is "aimed at any user who needs to write a config
file".

Change-Id: I698207fe58d564c615a15b50756aa5547a5f40b7
Signed-off-by: Yasushi SHOJI <yashi@spacecubics.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6737
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>