dev: ui_resource: Clean up implementation of set_deep_meta_attr
doc: Document revised nvpair reference syntax
high: parse: Revised syntax for nvpair references (fate#316118)

New syntax is as described in documentation updated in separate patch:

To explicitly set the id of an nvpair: $<id>:<name>=<value>
To refer to an nvpair: @<id>
To refer and rename: @<id>:<name>