analyzer: code is inactive due to #[cfg] directives: doc is disabled

The inactive-code check appears to false positive (IMO) on #[cfg(doc)].

Sample code

#[cfg(doc)]
use std::future::Future;

/// Handy struct that pairs well with [`Future`].
pub struct Past;

One could potentially argue that this is behaving-as-designed, since it's analyzing the code, but not the docs - but I found it a bit un-useful.

Some possible proposed solutions

• not include cfg(doc) code blocks in the inactive-code analyzer.
• Include docstrings in the analyzer
  Open to other ideas to solve.

Start with the provided #[cfg(doc)] reproduction and trace how the inactive-code analyzer handles cfg directives and documentation links. Compare the proposed options, then establish the expected behavior and add a regression test covering the sample code and its [Future] reference.