docs: improve (describe|test).only description (#1826)

* docs: improve (describe|test).only description

* Apply suggestions from code review

Co-authored-by: Vladimir <sleuths.slews0s@icloud.com>

docs/api/index.md

@@ -102,6 +102,13 @@ In Jest, `TestFunction` can also be of type `(done: DoneCallback) => void`. If t
   })
   ```
 
+  Sometimes it is very useful to run `only` tests in a certain file, ignoring all other tests from the whole test suite, which pollute the output.
+
+  In order to do that run `vitest` with specific file containing the tests in question.
+  ```
+  # vitest interesting.test.ts
+  ```
+
 ### test.concurrent
 
 - **Type:** `(name: string, fn: TestFunction, timeout?: number) => void`
@@ -290,6 +297,13 @@ When you use `test` in the top level of file, they are collected as part of the
   })
   ```
 
+  Sometimes it is very useful to run `only` tests in a certain file, ignoring all other tests from the whole test suite, which pollute the output.
+
+  In order to do that run `vitest` with specific file containing the tests in question.
+  ```
+  # vitest interesting.test.ts
+  ```
+
 ### describe.concurrent
 
 - **Type:** `(name: string, fn: TestFunction, timeout?: number) => void`