docs: add declare keyword

Author: mascii

docs/api/formsField.md

@@ -6,7 +6,7 @@ This function returns `FormsField` object.
 The `formsField()` takes 4 parameters:
 
 ```typescript
-function formsField<T extends (arg: any) => Form>(
+declare function formsField<T extends (arg: any) => Form>(
   generateForm: T,
   initialValueListOrLength: InitialValueListOrLength<T> = [],
   schema?: FormsFieldSchema<ReturnType<T>>,

docs/api/isValidForm.md

@@ -6,7 +6,7 @@ This function returns boolean.
 The `isValidForm()` takes 1 parameter:
 
 ```typescript
-function isValidForm<T extends Form>(
+declare function isValidForm<T extends Form>(
   form: T | (T & { $key: number })
 ): boolean;
 ```

docs/api/toObject.md

@@ -6,7 +6,7 @@ This function does not check if the form is valid.
 The `toObject()` takes 1 parameter:
 
 ```typescript
-function toObject<T extends Form>(
+declare function toObject<T extends Form>(
   form: T | (T & { $key: number })
 ): Expand<ToObjectOutput<T>>;
 ```