Update PGP/MIME extension examples

In eebf7be4, two example topics for PGP/MIME extensions were added: one-pass signature verification, and context-sensitive cryptography.

The first one, however, was discussed in the IETF interim meeting, and nobody spoke up to say they're doing one-pass signature verification. And since we already removed the SaltedHash header (which had a similar purpose), I don't think we should have this in the charter unless someone speaks up in favor of needing this.

Conversely, the original topic I meant to refer to with "PGP/MIME extensions", which I mentioned at IETF 117, is about PGP/MIME messages composed of multiple encrypted parts. However, there was no link to describe this topic, but now I've created an issue for it: rfc4880bis#171. So, I've added a link to that to replace the example of one-pass signature verification.

(Regarding context-sensitive cryptography, I think it's not purely related to PGP/MIME, and could also benefit other uses of OpenPGP. But OK, the categorization is not the most important thing, and I'm favor of the topic in general, obviously, so I've left that as-is.)