fix(AIP-134): add full replace usage caveats

aip/general/0134.md

@@ -105,6 +105,11 @@ message UpdateBookRequest {
     populated (have a non-empty value).
   - Update masks **must** support a special value `*`, meaning full replacement
     (the equivalent of `PUT`).
+    - API producers need to be concious of how adding new, mutable fields to a
+      resource will be handled when consumers use `*` without knowledge of said
+      new, mutable fields. Likewise consumers need to be concious of using `*`
+      only when the risks of doing so are acceptable. In general, it is safest
+      to explicitly specify the fields to update rather than use `*`.
 - The request message **must not** contain any other required fields, and
   **should not** contain other optional fields except those described in this
   or another AIP.