docs(thread_aware): gate the authoring guide with cfg(any(doc, test))

Addresses review feedback on PR #742: export the `_documentation` module only
for rustdoc and tests, matching the established pattern in `recoverable` and
`fetch`, so it does not become part of the crate's public API in normal builds.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: psandana

crates/thread_aware/src/lib.rs

@@ -178,6 +178,7 @@ mod wrappers;
 
 /// A guide to authoring thread-aware types: how to implement, test, and debug them, and the
 /// anti-patterns to avoid. See [the guide](_documentation).
+#[cfg(any(doc, test))]
 pub mod _documentation;
 
 pub mod closure;