Remove note that sync.Cond, sync.Mutex, sync.RWMutex and atomic.Value
can be created as part of other data structures. Structs can be embedded
by default, and default should not be repeated.

Fixes #20471.

Change-Id: If3f5703d3f60abd96482b58ca43450d02a9e645a
Reviewed-on: https://go-review.googlesource.com/44071Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>