Reverts part of CL 6460082.

If a doc comment describes a type by explaining the
meaning of one instance of the type, a leading article
is fine and makes the text less awkward.

Compare:
// A dog is a kind of animal.
// Dog is a kind of animal.

R=golang-dev, dsymonds, dvyukov, r
CC=golang-dev
https://golang.org/cl/6494066