include: Added \threadsafety notes to about 60 more functions.

Reference Issue #7140.

include/SDL3/SDL_platform.h

@@ -51,6 +51,8 @@ extern "C" {
  * \returns the name of the platform. If the correct platform name is not
  *          available, returns a string beginning with the text "Unknown".
  *
+ * \threadsafety It is safe to call this function from any thread.
+ *
  * \since This function is available since SDL 3.2.0.
  */
 extern SDL_DECLSPEC const char * SDLCALL SDL_GetPlatform(void);