docs: fix html anchor xref links

makeinfo has a long outstanding bug that means @anchors are not correctly
formatted for split html, see:
http://lists.gnu.org/archive/html/bug-texinfo/2012-06/msg00000.html

The issue relates to using spaces or hyphens in the @anchor name.
Issue also reported via Trac #44

Change-Id: Id72e23375dd167674b2ae5b314e8242b90a72a5f
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/1244
Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>