Introduce formal policy for APIs that return strings.

This declares that any `const char *` returned from SDL is owned by SDL, and
promises to be valid _at least_ until the next time the event queue runs, or
SDL_Quit() is called, even if the thing that owns the string gets destroyed
or changed before then.

This is noted in the headers as "the SDL_GetStringRule", so this will both be
greppable to find a detailed explaination in docs/README-strings.md and
wikiheaders will automatically turn it into a link we can point at the
appropriate documentation.

Fixes #9902.

(and several FIXMEs, both known and yet-undocumented.)

include/SDL3/SDL_sensor.h

@@ -158,6 +158,8 @@ extern SDL_DECLSPEC SDL_SensorID *SDLCALL SDL_GetSensors(int *count);
 /**
  * Get the implementation dependent name of a sensor.
  *
+ * The returned string follows the SDL_GetStringRule.
+ *
  * \param instance_id the sensor instance ID
  * \returns the sensor name, or NULL if `instance_id` is not valid
  *
@@ -224,6 +226,8 @@ extern SDL_DECLSPEC SDL_PropertiesID SDLCALL SDL_GetSensorProperties(SDL_Sensor
 /**
  * Get the implementation dependent name of a sensor.
  *
+ * The returned string follows the SDL_GetStringRule.
+ *
  * \param sensor The SDL_Sensor object
  * \returns the sensor name, or NULL if `sensor` is NULL.
  *