doc: attach note for deprecations

[jsha]

I noticed at https://docs.rs/rustls-webpki/latest/webpki/struct.TlsClientTrustAnchors.html that the type and its field are marked with #[deprecated(since = "0.101.2")]. There's an additional parameter to the deprecated attribute, called note:

https://doc.rust-lang.org/reference/attributes/diagnostics.html#the-deprecated-attribute

note — Specifies a string that should be included in the deprecation message. This is typically used to provide an explanation about the deprecation and preferred alternatives.

It would be useful to attach a note to this deprecation (and others that don't have notes) explaining why it was deprecated and what to use instead.

[cpu]

Thanks for pointing out that oversight. There were only two deprecations missing a note, but I think we could do a better job across the board on providing context for the changes.

Since the deprecated content is already removed in main we'll need a maintenance release to fix this up, I started one with updated note's in #149

[djc]

Just to connect the dots here, the offending changes here are from my PR #133.

[cpu]

This was resolved with #149 and the published v0.101.3 release. Thanks!