improve extending matchers docs

- add satisfies keyword as it improves types
- change ambient declaration file to how it is configured inside [testing lib](https://github.com/testing-library/jest-dom/blob/main/types/vitest.d.ts) since the current way didn't work for me
- rename MatcherResult to ExpectationResult as this is the current name of custom matcher return type according to source code (see MatchersObject > RawMatcherFn > ExpectationResult)

docs/guide/extending-matchers.md

@@ -11,6 +11,8 @@ This guide will explore extending matchers with `expect.extend`. If you are inte
 To extend default matchers, call `expect.extend` with an object containing your matchers.
 
 ```ts
+import type { MatchersObject } from '@vitest/expect'
+
 expect.extend({
   toBeFoo(received, expected) {
     const { isNot } = this
@@ -20,19 +22,17 @@ expect.extend({
       message: () => `${received} is${isNot ? ' not' : ''} foo`
     }
   }
-})
+} satisfies MatchersObject)
 ```
 
 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'
-
 interface CustomMatchers<R = unknown> {
   toBeFoo: () => R
 }
 
-declare module 'vitest' {
+declare module '@vitest/expect' {
   interface Assertion<T = any> extends CustomMatchers<T> {}
   interface AsymmetricMatchersContaining extends CustomMatchers {}
 }
@@ -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