Improve pull_request_template.md (#22888)

Update `pull_request_template.md` because:

- It's a kind idea to hide the tips. However, it's easier to include
them in the commit message by mistake when you cannot see them. Check
`git log | grep 'Please check the following:'`. So don't hide it, expose
it and help fix it.
- "for backports" is much clearer than "for bug fixes". I saw someone
post a PR to a release branch because they believed it was the right way
for a bugfix.
- "Allow edits by maintainers", or we have to ask the contributor to
update the branch and they could be confused.
- Remind the contributor that the words could be included in the commit
message, to avoid some words like "Hello", "Sorry". If they really need
them, they can separate them with a line, like:

```markdown
Close #xxxx
Because ... Then ... Finally ...
---
Hello, this is my first time opening a pull request. Sorry for any mistakes.
```
And the merger should be careful, check and delete the extra content
before merging.

---------

Co-authored-by: techknowlogick <[email protected]>

.github/pull_request_template.md

@@ -1,9 +1,9 @@
-<!--
-
+<!-- start tips --> 
 Please check the following:
-
-1. Make sure you are targeting the `main` branch, pull requests on release branches are only allowed for bug fixes.
-2. Read contributing guidelines: https://github.com/go-gitea/gitea/blob/main/CONTRIBUTING.md
-3. Describe what your pull request does and which issue you're targeting (if any)
-
--->  
+1. Make sure you are targeting the `main` branch, pull requests on release branches are only allowed for backports.
+2. Make sure you have read contributing guidelines: https://github.com/go-gitea/gitea/blob/main/CONTRIBUTING.md .
+3. Describe what your pull request does and which issue you're targeting (if any).
+4. It is recommended to enable "Allow edits by maintainers", so maintainers can help more easily.
+5. Your input here will be included in the commit message when this PR has been merged. If you don't want some content to be included, please separate them with a line like `---`.
+6. Delete all these tips before posting.
+<!-- end tips -->