Fix and extend anonymize_info

[cbrnr]

Fixes #13637.

In this PR, I have

• extended the keep_his parameter to accept True, False, and the strings "his_id", "sex", and "hand" (or a sequence of any combination of these strings),
• fixed the docs to reflect the actual behavior, and
• added new test cases (and adapted bits of existing tests).

Importantly, the default behavior doesn't change (keep_his=False), and keep_his=True keeps the three fields (also as previously, but this time I've documented it correctly).

[cbrnr]

In which category should I create the corresponding changelog entry?

[larsoner]

I think it counts as an enhancement (better control/granularity, no behavior change or deprecation)

[larsoner]

Just two optional ideas other than needing a doc/changes/dev/13647.newfeature.rst

[cbrnr]

Thanks @larsoner! Unless I messed up the rST in the changelog entry (chances are >50% 😃), this should be ready to merge!