Auto merge of #10605 - yoav-lavi:patch-1, r=ehuss

Add caveat for covering features

This section explains the caveats and possible solutions for features, detailing the tooling needed to 100% cover such a project.

The need for this PR is based on a conversation with `@Eh2406.`

Moved this PR here based on `@Eh2406's` comment: rust-lang/reference#1195 (comment)

src/doc/src/reference/features.md

@@ -513,3 +513,9 @@ source and inspect it.
 
 [`cargo vendor`]: ../commands/cargo-vendor.md
 [cargo-clone-crate]: https://crates.io/crates/cargo-clone-crate
+
+### Feature combinations
+
+Because features are a form of conditional compilation, they require an exponential number of configurations and test cases to be 100% covered. By default, tests, docs, and other tooling such as [Clippy](https://github.com/rust-lang/rust-clippy) will only run with the default set of features.
+
+We encourage you to consider your strategy and tooling in regards to different feature combinations - Every project will have different requirements in conjunction with time, resources, and the cost-benefit of covering specific scenarios. Common configurations may be with / without default features, specific combinations of features, or all combinations of features.