minor symfony#20381 [github] Tweak PR template (nicolas-grekas)

This PR was merged into the 2.7 branch.

Discussion
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[github] Tweak PR template

| Q             | A
| ------------- | ---
| Branch?       | 2.7
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Related to symfony#20295

Commits
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2acaf5f [github] Tweak PR template

Author: fabpot

.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.
+-->