Add moduleNameMapper ordering note to the documentation (#5249)

* Add moduleNameMapper ordering note to the documentation

* Add changelog entry

Author: emileber

CHANGELOG.md

@@ -40,6 +40,7 @@
 
 * `[docs]` Describe the order of execution of describe and test blocks.
   ([#5217](https://github.com/facebook/jest/pull/5217), [#5238](https://github.com/facebook/jest/pull/5238))
+* `[docs]` Add a note on `moduleNameMapper` ordering. ([#5249](https://github.com/facebook/jest/pull/5249))
 
 ## jest 22.0.4
 

docs/Configuration.md

@@ -386,6 +386,9 @@ Example:
 }
 ```
 
+The order in which the mappings are defined matters. Patterns are checked one
+by one until one fits. The most specific rule should be listed first.
+
 _Note: If you provide module name without boundaries `^$` it may cause hard to
 spot errors. E.g. `relay` will replace all modules which contain `relay` as a
 substring in its name: `relay`, `react-relay` and `graphql-relay` will all be