docs(typescript): add notes about virtual context to Mongoose 6 migration and TypeScript virtuals docs

Fix #12806

Author: vkarpov15

docs/migrating_to_6.md

@@ -511,3 +511,22 @@ The following legacy types have been removed:
 * `HookDoneFunction`
 * `SchemaTypeOpts`
 * `ConnectionOptions`
+
+Mongoose 6 infers the document's type for `this` in virtual getters and setters.
+In Mongoose 5.x, `this` would be `any` in the following code.
+
+```ts
+schema.virtual('myVirtual').get(function() {
+  this; // any in Mongoose 5.x
+});
+```
+
+In Mongoose 6, `this` will be set to the document type.
+
+```ts
+const schema = new Schema({ name: String });
+
+schema.virtual('myVirtual').get(function() {
+  this.name; // string
+});
+```

docs/typescript/virtuals.md

@@ -82,4 +82,20 @@ const schema = new Schema<UserDoc, UserModel, UserVirtuals>({ // <-- and here
 schema.virtual('fullName').get(function() {
   return `${this.firstName} ${this.lastName}`;
 });
+```
+
+### Override the Type of `this` in Your Virtual
+
+In case the value of `this` in your virtual is incorrect for some reason, you can always override it using the generic parameter to `virtual()`.
+
+```ts
+interface MyCustomUserDocumentType {
+  firstName: string;
+  lastName: string;
+  myMethod(): string;
+}
+
+schema.virtual<MyCustomUserDocumentType>('fullName').get(function() {
+  return this.method(); // returns string
+});
 ```