docs: Improve {ISSUE,PULL_REQUEST}_TEMPLATEs, slightly

[erikmd]

• Kind: enhancement

Description

This patch is a follow-up of PR #453

Checklist

• Read the CONTRIBUTING.md guide and:
  • Use Atomic Commits so each commit gathers a single logical change
  • Use Conventional Commits regarding commit messages (needed by our release toolchain)

Note to maintainers

• Read this wiki page
• Make sure the PR has a milestone
• Assign yourself before merging
• We can squash-merge 1-commit PRs (use a header with a conventional-commit type, add a footer with Fix #… if need be)