include: More category documentation.

Reference Issue #11847.

include/SDL3/SDL_time.h

@@ -26,6 +26,14 @@ freely, subject to the following restrictions:
  * # CategoryTime
  *
  * SDL realtime clock and date/time routines.
+ *
+ * There are two data types that are used in this category: SDL_Time, which
+ * represents the nanoseconds since a specific moment (an "epoch"), and
+ * SDL_DateTime, which breaks time down into human-understandable components:
+ * years, months, days, hours, etc.
+ *
+ * Much of the functionality is involved in converting those two types to
+ * other useful forms.
  */
 
 #include <SDL3/SDL_error.h>