docs(material/datepicker): Added context to docs for date picker null…

… value #29385 (#29387)

* Added context for why the null value is there

* Fixed formatting

* removed extra *

(cherry picked from commit 39b15dc)

src/material/datepicker/datepicker-input-base.ts

@@ -56,7 +56,11 @@ export class MatDatepickerInputEvent<D, S = unknown> {
   }
 }
 
-/** Function that can be used to filter out dates from a calendar. */
+/**
+ * Function that can be used to filter out dates from a calendar.
+ * Datepicker can sometimes receive a null value as input for the date argument.
+ * This doesn't represent a "null date" but rather signifies that no date has been selected yet in the calendar.
+ */
 export type DateFilterFn<D> = (date: D | null) => boolean;
 
 /**