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/16826Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

AUTHORS

@@ -2,9 +2,11 @@
 # This file is distinct from the CONTRIBUTORS files.
 # See the latter for an explanation.
 
-# Names should be added to this file as
-#	Name or Organization <email address>
-# The email address is not required for organizations.
+# Names should be added to this file as one of
+#     Organization's name
+#     Individual's name <submission email address>
+#     Individual's name <submission email address> <email2> <emailN>
+# See CONTRIBUTORS for the meaning of multiple email addresses.
 
 # Please keep the list sorted.
 

CONTRIBUTORS

@@ -22,12 +22,13 @@
 # individual or corporate CLA was used.
 
 # Names should be added to this file like so:
-#     Name <email address>
+#     Individual's name <submission email address>
+#     Individual's name <submission email address> <email2> <emailN>
 #
-# An entry with two email addresses specifies that the
+# An entry with multiple email addresses specifies that the
 # first address should be used in the submit logs and
-# that the second address should be recognized as the
-# same person when interacting with Rietveld.
+# that the other addresses should be recognized as the
+# same person when interacting with Gerrit.
 
 # Please keep the list sorted.