Documentation update for span line/end/etc?

The documentation for `proc-macro2` still says:

```Available on crate feature span-locations only.
Get the starting line/column in the source file for this span.

This method requires the "span-locations" feature to be enabled.

When executing in a procedural macro context, the returned line/column are only meaningful if compiled with a nightly toolchain. The stable toolchain does not have this information available. When executing outside of a procedural macro, such as main.rs or build.rs, the line/column are always meaningful regardless of toolchain.
```

With similar disclaimers for `end`, `file` etc. Since the MSRV is the "latest stable compiler ABI", I imagine these should be updated, and the reference to the nightly toolchain deleted? I'd be happy to raise a PR if you would like.

Thanks for being such an amazing force in the Rust ecosystem!