Update serialization docs

[sondrelg]

Checklist

• I have checked the issues list
  for similar or identical bug reports.
• I have checked the pull requests list
  for existing proposed fixes.
• I have checked the commit log
  to find out if the bug was already fixed in the master branch.
• I have included all related issues and possible duplicate issues in this issue
  (If there are none, check this box anyway).

Related Issues and Possible Duplicates

Related Issues

• None

Possible Duplicates

• None

Description

The serialization docs include a section on msgpack:

Specifically I'm referring to

It’s very young however, and support should be considered experimental at this point.

It seems this sentence was added 12 years back in this commit, which seems to have been around version 2.2.0.

Suggestions

I think it'd be good to revise the docs. I don't think this is accurate any longer. Do you agree? 🙂

[open-collective-bot]

Hey @sondrelg 👋,
Thank you for opening an issue. We will get back to you as soon as we can.
Also, check out our Open Collective and consider backing us - every little helps!

We also offer priority support for our sponsors.
If you require immediate assistance please consider sponsoring us.

[sondrelg]

I think these should probably also mention that accept_content needs to be expanded for other serialization formats to work. Otherwise you'll see discarded tasks #6953

I'd be happy to create a PR with slight revisions if you agree.

[auvipy]

please proceed