doc: add uv_replace_allocator thread safety warning

PR-URL: https://github.com/libuv/libuv/pull/2815/
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

docs/src/misc.rst

@@ -233,6 +233,8 @@ API
                  sure the allocator is changed while no memory was allocated with
                  the previous allocator, or that they are compatible.
 
+    .. warning:: Allocator must be thread-safe.
+
 .. c:function:: void uv_library_shutdown(void);
 
     .. versionadded:: 1.38.0