Include docs under GPLv2.

Fixes #2470.  The docs were previously under an unspecified
free-with-copyright clause, which appears to allow such relicensing.
Anyway, Warner is OK with the change.

master/docs/index.rst

@@ -37,8 +37,13 @@ Indices and Tables
 Copyright
 =========
 
-Copyright Buildbot Team Members
+This documentation is part of Buildbot.
+
+Buildbot is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, version 2.
+
+This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+See the GNU General Public License for more details.
 
-Copying and distribution of this file, with or without modification, are
-permitted in any medium without royalty provided the copyright notice and this
-notice are preserved.
+You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+
+Copyright Buildbot Team Members