[MINOR][INFRA] Add a guide to clarify release/unreleased Spark versions of user-facing change in the Github PR template

### What changes were proposed in this pull request?

This PR proposes to add a guide to clarify the Spark version when describing "Does this PR introduce any user-facing change?".

### Why are the changes needed?

It seems confusing to write when the user facing changes happen within unreleased branches.

### Does this PR introduce _any_ user-facing change?

No, dev-only.

### How was this patch tested?

Manually tested in Github and it renders find as intended.

Closes apache#28403 from HyukjinKwon/minor-more-guide.

Authored-by: HyukjinKwon <[email protected]>
Signed-off-by: HyukjinKwon <[email protected]>

Author: HyukjinKwon

.github/PULL_REQUEST_TEMPLATE

@@ -29,9 +29,11 @@ Please clarify why the changes are needed. For instance,
 -->
 
 
-### Does this PR introduce any user-facing change?
+### Does this PR introduce _any_ user-facing change?
 <!--
+Note that it means *any* user-facing change including all aspects such as the documentation fix.
 If yes, please clarify the previous behavior and the change this PR proposes - provide the console output, description and/or an example to show the behavior difference if possible.
+If possible, please also clarify if this is a user-facing change compared to the released Spark versions or within the unreleased branches such as master.
 If no, write 'No'.
 -->