docs: add gerrit http password url

Add url to show where the http password is configured.
Also add note that password can also be saved to url git config.

Change-Id: I3c1a022580e5f73372b0c50e8d1d2f0b1498966f
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/1207
Tested-by: jenkins