reflect/protoreflect: ExtensionRangeOptions never returns nil
reflect/protoreflect: ExtensionRangeOptions never returns nil

Documentation only change.

Change-Id: Ib620efc99bdaab90eeb5404554ae442f345d6f47
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/221560
Reviewed-by: Joe Tsai <ad9e03fbec35106ba742f446a098eef3d5e31d20@google.com>

protocolbuffers/protobuf-go,protocolbuffers/protobuf-go