Document that the client/server features are now opt-in

[edmorley]

Adds documentation for the switch to the client and server features now being opt-in, rather than enabled by default (see #54).

I experimented with also enabling the nightly-only doc_cfg / doc_auto_cfg features (that allow showing feature labels in the docs, see rust-lang/rust/issues/43781), however:

• the features don't currently work well with the re-exports in the crate root, so require every struct/trait to be manually annotated.
• the rendering isn't great - the crate landing page wrapping ended up broken
• (to a lesser extent) whilst these can be used on docs.rs (since it builds docs with nightly), they won't work for the GitHub pages version of the docs this project uses to preview main.

So for now it seems best to not use that.

I did adjust the docs.rs config in Cargo.toml to ensure docs are actually generated for all features, since otherwise the docs were almost empty after #54.

I also fixed a few typos / markdown syntax errors (let me know if you'd prefer these in a separate PR).

[onalante-msft]

Sorry this took so long to review. I had this on my to-do and forgot about it.

[edmorley]

@onalante-msft No need to be sorry! Thank you for taking a look :-)

[edmorley]

We've since dropped this as a (transitive) dependency so are no longer using this crate. Closing the PR to clean up my open PRs list.