internal/jsonrpc2: add some extra comments
internal/jsonrpc2: add some extra comments

Change-Id: I31531f32a88a02acf82814ab4ea151e54fd6d0fc
Reviewed-on: https://go-review.googlesource.com/c/tools/+/184037
Reviewed-by: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools