reflect/protoreflect: clarify Mutable semantics
reflect/protoreflect: clarify Mutable semantics

The Mutable semantics are simpler than currently documented. In an older design
of the protobuf reflection API, it was intended that there could be a mutable
version of the scalar kinds. However, support for this has been dropped,
simplifying the documented semantics.

Change-Id: If66740ffa5da65a3f2c8cf46700e0c23393a16d7
Reviewed-on: https://go-review.googlesource.com/c/144857
Reviewed-by: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>

protocolbuffers/protobuf-go,protocolbuffers/protobuf-go