doc: mention strerror implementation details

docs/implementation-guide.md

@@ -54,4 +54,11 @@ When closing leaf link in a chain:
    by one
 3. `close_cb` will be invoked upon close of all links in a chain
 
+## uv_link_strerror()
+
+Implementation of `uv_link_methods_t.strerror` MUST check that the passed error
+was emitted by the `link` argument. If this is not the case -
+`uv_link_strerror(link->parent, err)` MUST be called (in other words, call
+MUST be propagated).
+
 [0]: http://docs.libuv.org/en/v1.x/stream.html