yang: handle description deviations

[troglobit]

Occasionally we wish to make a deviation for a "description" of a leaf in a standard yang model, typically when our implementation has limitations. Keeping description up to date with the functionality we support is good as we can use the supported yang models (with deviations) as Infix documentation; and to do this in one place.

However, deviations for description is unfortunately not possible today, and waiting for a new yang version (1.2) is not realistic.
See the following reported yang issue for more info: Allow deviation for description.

What work-arounds do we have? @wkz mentioned extending yang with another meta-data(?), where we could write updated documentation. If that "field" exists, we could parse that instead (when compiling doc offline or when showing "help cmd" on target)

[troglobit]

CCB: We could have an extra YANG model that is added only for the yang-doc.html generation.