[@types/jest] Improve toContain matcher description. (#42965)

types/jest/index.d.ts

@@ -26,6 +26,7 @@
 //                 Tony Hallett <https://github.com/tonyhallett>
 //                 Jason Yu <https://github.com/ycmjason>
 //                 Devansh Jethmalani <https://github.com/devanshj>
+//                 Pawel Fajfer <https://github.com/pawfa>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
 // TypeScript Version: 3.0
 
@@ -805,6 +806,7 @@ declare namespace jest {
         /**
          * Used when you want to check that an item is in a list.
          * For testing the items in the list, this uses `===`, a strict equality check.
+         * It can also check whether a string is a substring of another string.
          *
          * Optionally, you can provide a type for the expected value via a generic.
          * This is particuarly useful for ensuring expected objects have the right structure.

types/jest/jest-tests.ts

@@ -869,6 +869,7 @@ describe('', () => {
         expect([]).toContain({});
         expect(['abc']).toContain('abc');
         expect(['abc']).toContain('def');
+        expect('abc').toContain('bc');
 
         expect([]).toContainEqual({});
         expect(['abc']).toContainEqual('def');