doc: Clarification about GPLv2+/GPLv2 licensing

Avocado itself is released under GPLv2 (explicit version), but most of the code
is currently licensed under GPLv2+, so make that clear in the documentation.
Signed-off-by: NAdemar de Souza Reis Jr <areis@redhat.com>

LICENSE

-avocado is released specifically under the GPLv2, and not any
-later version of the license, unless explicitly otherwise stated
-
-All files in the avocado project are covered by the GPLv2, unless
-explicitly otherwise stated.
-
+Most files in Avocado are released under the GPLv2 with a "and any later
+version" clause (GPLv2+), but some files are explicitly bound to the
+version 2 of the license and therefore that's the official license of
+the project itself. The header on each individual file contains
+information about the GPL version that applies to it.
+
+Unless explicitly otherwise stated, all files in the avocado project
+are covered by the GPLv2+ (i.e: v2, or any later version).
 
                    GNU GENERAL PUBLIC LICENSE
                       Version 2, June 1991

man/avocado.rst

@@ -491,6 +491,16 @@ BUGS
 
 If you find a bug, please report it over our github page as an issue.
 
+LICENSE
+================
+
+Avocado is released under GPLv2 (explicit version)
+`http://gnu.org/licenses/gpl-2.0.html`. Even though most of the current code is
+licensed under a "and any later version" clause, some parts are specifically
+bound to the version 2 of the license and therefore that's the official license
+of the prject itself. For more details, please see the LICENSE file in the
+project source code directory.
+
 MORE INFORMATION
 ================