Update the documentation of ignore (rust-lang#3580)

Author: topecongiro

Configurations.md

@@ -2325,7 +2325,8 @@ Copyright 2018 The Rust Project Developers.`, etc.:
 
 ## `ignore`
 
-Skip formatting the specified files and directories.
+Skip formatting files and directories that match the specified pattern.
+The pattern format is the same as [.gitignore](https://git-scm.com/docs/gitignore#_pattern_format).
 
 - **Default value**: format every file
 - **Possible values**: See an example below
@@ -2350,6 +2351,12 @@ ignore = [
 ]
 ```
 
+If you want to ignore every file under the directory where you put your rustfmt.toml:
+
+```toml
+ignore = ["/"]
+```
+
 ## `edition`
 
 Specifies which edition is used by the parser.