[github] Tweak PR template

Author: nicolas-grekas

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,11 +1,19 @@
 | Q             | A
 | ------------- | ---
-| Branch?       | "master" for new features / 2.7, 2.8 or 3.1 for fixes
+| Branch?       | master / 2.7, 2.8, 3.1 or 3.2 <!--see comment below-->
 | Bug fix?      | yes/no
 | New feature?  | yes/no
 | BC breaks?    | yes/no
 | Deprecations? | yes/no
 | Tests pass?   | yes/no
-| Fixed tickets | comma-separated list of tickets fixed by the PR, if any
+| Fixed tickets | #... <!-- #-prefixed issue number(s), if any -->
 | License       | MIT
-| Doc PR        | reference to the documentation PR, if any
+| Doc PR        | symfony/symfony-docs#... <!--highly recommended for new features-->
+
+<!--
+- Bug fixes must be submitted against the lowest branch where they apply
+  (lowest branches are regularly merged to upper ones so they get the fixes too).
+- Features and deprecations must be submitted against the master branch.
+- Please fill in this template according to the PR you're about to submit.
+- Replace this comment by a description of what your PR is solving.
+-->