docs: add RTOS Support info

Currently we have very little info about the RTOS support. This should
improve that.

We also add info about what symbols are required for each supported RTOS.
This can be a trap, certainly when trying to use FreeRTOS support.

Change-Id: Ie57858571daca97515292ff5738a5a5ef55655b7
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/1538
Tested-by: jenkins