Clarify exception: avoid embedding mutexes in all cases (uber-go#138)

Author: mway

style.md

@@ -2593,9 +2593,11 @@ type Client struct {
 Embedding should provide tangible benefit, like adding or augmenting
 functionality in a semantically-appropriate way. It should do this with zero
 adverse user-facing effects (see also: [Avoid Embedding Types in Public Structs]).
-Generally, mutexes should not be embedded unless they are an integral part of the embedding type's API.
+
+Exception: Mutexes should not be embedded, even on unexported types. See also: [Zero-value Mutexes are Valid].
 
   [Avoid Embedding Types in Public Structs]: #avoid-embedding-types-in-public-structs
+  [Zero-value Mutexes are Valid]: #zero-value-mutexes-are-valid
 
 Embedding **should not**: