chips/contactChips: md-on-append and md-require-match need better docs

[topherfangio]

Currently, the docs for md-on-append and md-require-match do not explain very well how they work alone or when mixed together.

Need to provide some more info/usage snippets and potentially a demo to clarify.

Should wait until PRs #3816 and #4479 are finished/merged so functionality works better (particularly, you should be ale to use both and have md-on-append return null to disallow adding a chip, but this doesn't work yet).

[JordanMajd]

+1

[tonyawad88]

+1

[e-oz]

related: #4193

[Frank3K]

+1 for future users. Just spent some time debugging until I connected the dots on md-require-match.