proto: simplify the documentation
proto: simplify the documentation

The current docstring is unnecessarily complicated.
Simply mention what the package is primarily for and then point
users to a better written manual on using Go protobufs.

Change-Id: I63d0580d1573ebfc1dc88b8bad326d7f1e030b88
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/167765
Reviewed-by: Herbie Ong <86f376bf42b6daa37c768c730acd318ab6855de4@google.com>

golang/protobuf,golang/protobuf