docs: add MonoTypeOperatorFunction documentation (#7284)

* docs: add MonoTypeOperatorFunction documentation

* docs: update MonoTypeOperatorFunction documentation

Co-authored-by: Mladen Jakovljević <jakovljevic.mladen@gmail.com>

---------

Co-authored-by: Mladen Jakovljević <jakovljevic.mladen@gmail.com>
(cherry picked from commit 0fe6b2f)

Author: arahm730

src/internal/types.ts

@@ -31,6 +31,12 @@ export interface OperatorFunction<T, R> extends UnaryFunction<Observable<T>, Obs
 
 export type FactoryOrValue<T> = T | (() => T);
 
+/**
+ * A function type interface that describes a function that accepts and returns a parameter of the same type.
+ *
+ * Used to describe {@link OperatorFunction} with the only one type: `OperatorFunction<T, T>`.
+ *
+ */
 export interface MonoTypeOperatorFunction<T> extends OperatorFunction<T, T> {}
 
 /**