btvars and rtvars help

Feature request:

Currently the overlays/layers have a description which describe various btvars and rtvars. These btvars and rtvars are inherited by the configs which include them, but we lose the documentation for them as the description of overlay/layer is not propagated when the config is inspected. If the btvars and rtvars can also have help field which has a small description, then as part of inheriting these variable, it may be possible to inherit their help fields as well and display them during inspect.

This should just be a case of defining a description field for btvars and rtvars, then refactoring parts of the global description into those new fields for each existing config. Finally modify inspect to parse and display those fields in its output.