client API: rename mpv_destroy() to mpv_detach_destroy()

A bit verbose, but less misleading. In most cases, the API user probably
actually wants mpv_terminate_destroy() instead, so the less-useful
function shouldn't have a simnpler name anyway.

DOCS/client_api_examples/simple.c

@@ -39,6 +39,6 @@ int main(int argc, char *argv[])
             break;
     }
 
-    mpv_destroy(ctx);
+    mpv_terminate_destroy(ctx);
     return 0;
 }