Release process: clarify how to set the MAJOR_RELEASE environment variable when preparing a RC.

wiki/src/contribute/release_process.mdwn

@@ -38,7 +38,8 @@ the scripts snippets found on this page:
 * `NEXT_PLANNED_MINOR_VERSION`: set to the version number of the next
   *minor* Tails release; if the next release is a point-release, use
   that one; otherwise, use `${VERSION}.1`
-* `MAJOR_RELEASE`: set to 1 if preparing a major release, to 0 else
+* `MAJOR_RELEASE`: set to 1 if preparing a major release or a release
+  candidate for a major release, to 0 otherwise
 * `ISOS`: the directory where one stores `tails-amd64-*`
   sub-directories like the ones downloaded with BitTorrent.
 * `ARTIFACTS`: the directory where build artifacts (e.g.