Grammatical cleanup to Tcl Scripting chapter of User's Guide

Grammatical cleanup to Tcl Scripting chapter of User's Guide

Changes solely to the Tcl Scripting API chapter of the UG:

* Some grammatical cleanup
* Fix formatting issues (@example and @verbatim formatting)
* Add references to missing OSes OpenBSD, NetBSD and eCos.

Change-Id: I3ec1a192a0b1e0a207dceb76fd39008d01e287a5
Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Reviewed-on: http://openocd.zylin.com/1872
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>