Document usage guidelines for DomUtils.getElementsByTagName*

Adds documentation as requested. Fixed #736.
Optimize all existing uses of getElementsByTagNameNS

Transformed all read-only uses to use node.getElements*. Any loop that
immediately modifies the nodes in the list has been updated to use the
DomUtils.getElements* version.
Merge pull request #792 from peitschie/getelementbyns

Document guidelines for getElementsByTagNameNS and optimize all usages