Chapter for param_attrs

src/doc/unstable-book/src/language-features/param-attrs.md

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+# `param_attrs`
+
+The tracking issue for this feature is: [#60406]
+
+[#60406]: https://github.com/rust-lang/rust/issues/60406
+
+Allow attributes in formal function parameter position so external tools and compiler internals can
+take advantage of the additional information that the parameters provide.
+
+Enables finer conditional compilation with `#[cfg(..)]` and linting control of variables. Moreover,
+opens the path to richer DSLs created by users.
+
+------------------------
+
+Example:
+
+```rust
+#![feature(param_attrs)]
+
+fn len(
+  #[cfg(windows)] slice: &[u16],
+  #[cfg(not(windows))] slice: &[u8],
+) -> usize
+{
+  slice.len()
+}
+```