[SPARK-50860][INFRA] Clarify Exclusion of Doc-Only Updates from User-Facing Changes

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

This PR proposes updating the Github PR template to clarify that documentation-only updates are not considered user-facing changes.
### Why are the changes needed?

As discussed in https://lists.apache.org/thread/mld03dz9n6xh7pp21wtpvczl9pbmwpm9, the current PR template considers documentation-only updates as user-facing changes, which is not the intent and can cause confusion.

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

No

### How was this patch tested?

It's just template change

### Was this patch authored or co-authored using generative AI tooling?

No

Closes #49534 from gengliangwang/updateGithubTemplate.

Authored-by: Gengliang Wang <gengliang@apache.org>
Signed-off-by: Hyukjin Kwon <gurwls223@apache.org>