fnsince: update \since policy for documentation.

Everything in SDL3 up to the ABI lock is reported as available since 3.1.3.
Everything else will be reported as since 3.2.0 (what will be the first
official release).

Also ran a Perl script over the headers to change everything to 3.1.3 that
wasn't an API function, since fnsince.pl can't manage those. If there's a
macro or datatype that has snuck in that needs to be 3.2.0 instead, we'll
have to manually fix it up, but it shouldn't be a big deal in any case.

Reference PR #11304.

include/SDL3/SDL_power.h

@@ -42,7 +42,7 @@ extern "C" {
  *
  * These are results returned by SDL_GetPowerInfo().
  *
- * \since This enum is available since SDL 3.0.0
+ * \since This enum is available since SDL 3.1.3
  */
 typedef enum SDL_PowerState
 {
@@ -80,7 +80,7 @@ typedef enum SDL_PowerState
  * \returns the current battery state or `SDL_POWERSTATE_ERROR` on failure;
  *          call SDL_GetError() for more information.
  *
- * \since This function is available since SDL 3.0.0.
+ * \since This function is available since SDL 3.1.3.
  */
 extern SDL_DECLSPEC SDL_PowerState SDLCALL SDL_GetPowerInfo(int *seconds, int *percent);