Proposal for clarifications when calling gNMI Set on a list node or element.

[wenovus]

1. Add example JSON_IETF format for updating an individual list element.
2. Clarify that an empty subset of attributes for a keyed-list is unacceptable as a path for gNMI Set.
3. Clarify that in OpenConfig, MUST specify a path that terminates on the enclosing container in order to modify a whole list node.
4. Minor grammar fixes.

Background

I found these points to be missing from either this doc or RFC7951 (#139). After some discussions with Anees, I'm proposing to add these clarifications that may or may not be the standard or established convention. In that case, modification suggestions are welcome to correct any inaccuracies.