Stabilize ignore config option (#4139)

Author: topecongiro

Configurations.md

@@ -1051,7 +1051,7 @@ The pattern format is the same as [.gitignore](https://git-scm.com/docs/gitignor
 
 - **Default value**: format every file
 - **Possible values**: See an example below
-- **Stable**: No (tracking issue: [#3395](https://github.com/rust-lang/rustfmt/issues/3395))
+- **Stable**: Yes
 
 ### Example
 

rustfmt-core/rustfmt-lib/src/config.rs

@@ -149,7 +149,7 @@ create_config! {
     error_on_unformatted: bool, false, false,
         "Error if unable to get comments or string literals within max_width, \
          or they are left with trailing whitespaces";
-    ignore: IgnoreList, IgnoreList::default(), false,
+    ignore: IgnoreList, IgnoreList::default(), true,
         "Skip formatting the specified files and directories";
 
     // Not user-facing