cmd/doc: rearrange the newlines to group better

cmd/doc: rearrange the newlines to group better

Main change is that the comment for an item no longer has a blank line
before it, so it looks bound to the item it's about.

Motivating example: go doc.io.read changes from

<
func (l *LimitedReader) Read(p []byte) (n int, err error)
func (r *PipeReader) Read(data []byte) (n int, err error)

    Read implements the standard Read interface: it reads data from the pipe,
    blocking until a writer arrives or the write end is closed. If the write end
    is closed with an error, that error is returned as err; otherwise err is
    EOF.
func (s *SectionReader) Read(p []byte) (n int, err error)
>

to

<
func (l *LimitedReader) Read(p []byte) (n int, err error)
func (r *PipeReader) Read(data []byte) (n int, err error)
    Read implements the standard Read interface: it reads data from the pipe,
    blocking until a writer arrives or the write end is closed. If the write end
    is closed with an error, that error is returned as err; otherwise err is
    EOF.

func (s *SectionReader) Read(p []byte) (n int, err error)
>

Now the comment about PipeReader.Read doesn't look like it's about
SectionReader.

Based on a suggestion by dsnet@, a slight tweak from a CL he suggested
and abandoned.

Fixes #12756,

Change-Id: Iaf60ee9ae7f644c83c32d5e130acab0312b0c926
Reviewed-on: https://go-review.googlesource.com/14999
Reviewed-by: Andrew Gerrand <adg@golang.org>