docs: Improve {ISSUE,PULL_REQUEST}_TEMPLATEs, slightly

This patch is a follow-up of PR ocaml-sf#453

Author: erikmd

.github/ISSUE_TEMPLATE/bug_report.md

@@ -40,9 +40,11 @@ Steps to reproduce the behavior:
 
 ## Current configuration
 
+<!-- You can write "N/A" for the items that sound irrelevant to you -->
 * OS name (and version): 
 * Browser name (and version): 
 * `learn-ocaml --version`: 
+* `git describe --long --always --abbrev=40`: 
 
 ## Additional context
 

.github/PULL_REQUEST_TEMPLATE.md

@@ -28,3 +28,4 @@
 * Read [this wiki page](https://github.com/ocaml-sf/learn-ocaml/wiki/Checklist-for-testing-and-merging-a-PR)
 * Make sure the PR has a milestone
 * Assign yourself before merging
+* We can squash-merge 1-commit PRs (ensuring the header has a [conventional-commit type](https://github.com/ocaml-sf/learn-ocaml/blob/master/CONTRIBUTING.md#conventional-commits-examples) and the footer says `Close #…`)