Rollup merge of rust-lang#88057 - ehuss:releases-doc-macros, r=Mark-Simulacrum

Update RELEASES to clarify attribute macro values.

As noted in rust-lang#87681, macros do not work with the `#[path]` attribute.  Since the places where macros *can* be used is very limited, I have changed this to just focus on `#[doc]` which is the only attribute where this is really useful.

Author: camsteffen

RELEASES.md

@@ -146,18 +146,13 @@ Version 1.54.0 (2021-07-29)
 Language
 -----------------------
 
-- [You can now use macros for values in built-in attribute macros.][83366]
-  While a seemingly minor addition on its own, this enables a lot of
-  powerful functionality when combined correctly. Most notably you can
-  now include external documentation in your crate by writing the following.
+- [You can now use macros for values in some built-in attributes.][83366]
+  This primarily allows you to call macros within the `#[doc]` attribute. For
+  example, to include external documentation in your crate, you can now write
+  the following:
   ```rust
   #![doc = include_str!("README.md")]
   ```
-  You can also use this to include auto-generated modules:
-  ```rust
-  #[path = concat!(env!("OUT_DIR"), "/generated.rs")]
-  mod generated;
-  ```
 
 - [You can now cast between unsized slice types (and types which contain
   unsized slices) in `const fn`.][85078]