Document some structure fields as public

[gilles-peskine-arm]

#4511 systematically documents all structure fields as private.

But some structure fields should remain public. The goal of this task is to take care of fields that should obviously remain public.

• Lighten and test constraints on context types in alternative implementations #4595 documents several fields of mbedtls_ecp_group as public.
• mbedtls_x509_crt_profile only makes sense if all its fields are public. We should document explicitly that new versions of the library may add fields. Done by Make the fields of mbedtls_x509_crt_profile public #4671

[mpg]

In the past I've seen application code access the state field of mbedtls_ssl_context. I don't think we necessarily want to make that field public (as it could hinder work on 1.3 which has a different state machine), but we probably want to make some info public, for example "is the handshake over yet?", otherwise exposing mbedtls_ssl_handshake_step() in the API makes little sense.

[mpg]

I'm marking this issue as "size: S" and closed by #4671 as the description says this is about "obvious" fields and apparently this is the last "obvious" thing we notice. Further work in this area is tracked in #4383 and #4683 for the somewhat-known parts, and the mostly-unknown parts are represented by #4682.