crypto/tls: make it clear that implementation is compliant with RFC 5246

[ALTree]

In #31933, @swanandt pointed out that the following note at the top of the crypto/tls package:

Package tls partially implements TLS 1.2, as specified in RFC 5246

may be misinterpreted as a warning about non-compliance with the RFC 5246. This is not the case, as @FiloSottile explained:

As far as I know, we implement everything that is REQUIRED by RFC 5246, so we are in full compliance with it.

It may be useful to slightly change the wording of the doc to make it completely clear the only optional features (like DHE, topic of the discussion at #31933) are currently not implemented, and so the package is fully compliant with RFC 5246.

[gopherbot]

Change https://golang.org/cl/177898 mentions this issue: crypto/tls: clarify that all required features of TLS are supported