doc: add note about uv_run() not being reentrant

PR-URL: https://github.com/libuv/libuv/pull/2208
Refs: https://github.com/libuv/help/issues/81
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Saúl Ibarra Corretgé <saghul@gmail.com>

docs/src/loop.rst

@@ -107,6 +107,8 @@ API
       or requests left), or non-zero if more callbacks are expected (meaning
       you should run the event loop again sometime in the future).
 
+    :c:func:`uv_run` is not reentrant. It must not be called from a callback.
+
 .. c:function:: int uv_loop_alive(const uv_loop_t* loop)
 
     Returns non-zero if there are referenced active handles, active