Clarify generated marker requires a config option

Author: stepancheg

CHANGELOG.md

@@ -61,6 +61,7 @@ Note this hit the rustup distributions prior to the v1.4.38 release as part of a
 
 - New `One` variant added to `imports_granularity` configuration option which can be used to reformat all imports into a single use statement [#4669](https://github.com/rust-lang/rustfmt/issues/4669)
 - rustfmt will now skip files that are annotated with `@generated` at the top of the file [#3958](https://github.com/rust-lang/rustfmt/issues/3958)
+  if `format_generated_files` option is set to `false` (by default `@generated` files are formatted)
 - New configuration option `hex_literal_case` that allows user to control the casing utilized for hex literals [PR #4903](https://github.com/rust-lang/rustfmt/pull/4903)
 
 See the section on the configuration site for more information

Configurations.md

@@ -930,6 +930,8 @@ fn add_one(x: i32) -> i32 {
 
 Format generated files. A file is considered generated
 if any of the first five lines contain a `@generated` comment marker.
+By default, generated files are reformatted, i. e. `@generated` marker is ignored.
+This option is currently ignored for stdin (`@generated` in stdin is ignored.)
 
 - **Default value**: `true`
 - **Possible values**: `true`, `false`