Renamed SDL_ThreadID() to SDL_GetCurrentThreadID()

Also renamed SDL_threadID to SDL_ThreadID and made it Uint64 for consistency with other ID types

docs/README-migration.md

@@ -1406,6 +1406,10 @@ The following functions have been renamed:
 * SDL_TLSCreate() => SDL_CreateTLS()
 * SDL_TLSGet() => SDL_GetTLS()
 * SDL_TLSSet() => SDL_SetTLS()
+* SDL_ThreadID() => SDL_GetCurrentThreadID()
+
+The following symbols have been renamed:
+* SDL_threadID => SDL_ThreadID
 
 ## SDL_timer.h