docs: improve extending matchers docs (#6121)

Author: vorant94

docs/guide/extending-matchers.md

@@ -26,7 +26,7 @@ expect.extend({
 If you are using TypeScript, you can extend default `Assertion` interface in an ambient declaration file (e.g: `vitest.d.ts`) with the code below:
 
 ```ts
-import type { Assertion, AsymmetricMatchersContaining } from 'vitest'
+import 'vitest'
 
 interface CustomMatchers<R = unknown> {
   toBeFoo: () => R
@@ -45,7 +45,7 @@ Don't forget to include the ambient declaration file in your `tsconfig.json`.
 The return value of a matcher should be compatible with the following interface:
 
 ```ts
-interface MatcherResult {
+interface ExpectationResult {
   pass: boolean
   message: () => string
   // If you pass these, they will automatically appear inside a diff when