A+C: alter email-address description to cover more than two

Explictly list the alternative formats in each file.  In AUTHORS, refer
to CONTRIBUTORS' definition of multiple email addresses.  Indent with
four spaces;  AUTHORS used a tab, but CONTRIBUTORS used four spaces.
s/Rietveld/Gerrit/

Change the tab separating Sebastien Binet from his email address,
added in 2010's 18b02f6c.

Change-Id: Id52228ae6b62dd88ad8098110c22373bf14e068f
Reviewed-on: https://go-review.googlesource.com/16826


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>