Link to RFC8760

[skhrshin]

pjproject_docs/docs/source/overview/features_sip.rst

Line 15 in f877648

- Digest authentication (:rfc:`2617`)

pjsip seems to be supporting digest authentication with SHA-256. I think the document should refer to RFC8760 too.

[skhrshin]

Oh, pjsip didn't have server-side support of this. So some additional note describing that the support is partial will be needed.