go/doc, godoc: improved note reading

- A note doesn't have to be in the first
comment of a comment group anymore, and
several notes may appear in the same comment
group (e.g., it is fairly common to have a
TODO(uid) note immediately following another
comment).

- Define a doc.Note type which also contains
note uid and position info.

- Better formatting in godoc output. The position
information is not yet used, but could be used to
locate the note in the source text if desired.

Fixes issue 4843.

R=r, cnicolaou
CC=gobot, golang-dev
https://codereview.appspot.com/7496048