GitHub Templates: Pull Request Template update (#18300)

* Imperative mood

The English language suggests that when giving out instructions or considerations is best to use imperative mood as the writing style. Done as part of this commit:

- some worth smithing to match this style
- changed the format to match the fact that this is commonly read as part of a text box when opening up your PRs
- and extended a few descriptions to make it more friendly for newcomers 

.github/PULL_REQUEST_TEMPLATE.md

-<!--  Thanks for sending a pull request!  Here are some tips for you:
-
-1. If this is your first time, please read our [`CONTRIBUTING.md`](https://github.com/grafana/grafana/blob/master/CONTRIBUTING.md) guide.
-2. Ensure you have added or ran the appropriate tests for your PR.
-3. If it's a new feature or config option it will need a docs update. Docs are under the docs folder in repo root.
-4. If the PR is unfinished, mark it as a draft PR.
-5. Rebase your PR if it gets out of sync with master
-6. Name your PR as `<FeatureArea>: Describe your change`. If it's a fix or feature relevant for changelog describe the user  impact in the title. The PR title is used in changelog for issues marked with `add to changelog` label. 
+<!--
+
+Thank you for sending a pull request! Here are some tips:
+
+1. If this is your first time, please read our contribution guide at https://github.com/grafana/grafana/blob/master/CONTRIBUTING.md
+
+2. Ensure you include and run the appropriate tests as part of your Pull Request.
+
+3. In a new feature or configuration option, an update to the documentation is necessary. Everything related to the documentation is under the docs folder in the root of the repository.
+
+4. If the Pull Request is a work in progress, make use of GitHub's "Draft PR" feature and mark it as such.
+
+5. If you can not merge your Pull Request due to a merge conflict, Rebase it. This gets it in sync with the master branch.
+
+6. Name your PR as "<FeatureArea>: Describe your change", e.g. Alerting: Prevent race condition. If it's a fix or feature relevant for the changelog describe the user impact in the title. The PR title is used to auto-generate the changelog for issues marked with the "add to changelog" label.
+
 -->
 
 **What this PR does / why we need it**:
 
 **Which issue(s) this PR fixes**:
+
 <!--
-*Automatically closes linked issue when PR is merged.
-Usage: `Fixes #<issue number>`, or `Fixes (paste link of issue)`.
+
+* Automatically closes linked issue when the Pull Request is merged.
+
+Usage: "Fixes #<issue number>", or "Fixes (paste link of issue)"
+
 -->
+
 Fixes #
 
 **Special notes for your reviewer**: