Promote AIP-162 to approved

[andrei-scripniciuc]

Summary:

• Disallow Update standard method, as revisions don't expose mutable fields
• Fix code samples
• Promote AIP to approved

There are a few items that still need clarification, as called out in #1206:

• The user journey of creating a revision with a semantic name is a bit rough: you have to create a revision, then assign it an alias. Ideally it should be one call.
• Revisions also states that a revision could be a sibling collection instead of a child collection. This pattern is probably possible to support, the AIP needs to be rewritten to allow that (e.g. clarify what the custom method path should be in the case of siblings).
• How do we allow a single field to address "either a resource or a revision".

These changes can be incrementally added later, moving AIP to approved as we're not planning to address these gaps in the near future.

[jskeet]

I'm fine with everything apart from the change to making it approved.

[jskeet]

I'd prefer to separate "we're still making changes" from "it's ready from approval".

[noahdietz]

Perhaps set to reviewing?

[noahdietz]

LGTM but if there are things still in flux, let's put into reviewing