Clarify Apache Header always vs onsuccess behavior to prevent duplication (#2008)

[HimasreeKolathur24]

Summary

This PR clarifies the behavior of Header set (default onsuccess) and Header always set in Apache mod_headers.

Previously, the section mentioned the always option but did not explain that it operates on a separate internal header table. On 2xx responses, both onsuccess and always directives may apply, which can result in duplicate headers if the same header is configured in both contexts.

This update:

• Adds a short clarification about the separation of header tables.
• Warns about potential header duplication.
• Recommends a safe configuration pattern using Header unset before Header always set.

The goal is to prevent misconfiguration while keeping the guidance concise and practical.

This PR fixes issue #2008.

---

AI Tool Usage Disclosure

• I have used AI tools to generate the contents of this PR. I have verified the contents and I affirm the results. The LLM used is ChatGPT (GPT-5.2) and the prompt used was assistance in clarifying Apache mod_headers behavior and improving documentation wording.

[Brandon-Johns]

Thank you for working on this.
It's good.

I think it could be slightly improved with

• Restore the link to the Apache documentation to enable further reading, in case that the solution does not meet the needs of the reader (if they need to apply different values of the headers for different status codes).
• Add a note that to unset a header, it should be unset for both onsuccess and always.
• Given the edge cases, as I discussed in the issue thread, I suggest to alter the sentance
  • From: On 2xx responses, both may apply, which can result in duplicate headers if the same header is configured in both contexts.
  • To: In some cases, both header tables may be used, which can result in duplicate headers if the same header is configured in both contexts.

[HimasreeKolathur24]

@Brandon-Johns,
Thank you for the feedback. I’ve updated the section to restore the documentation link, adjust the wording, and clarify unsetting in both contexts. Please let me know if anything else should be refined.

[Brandon-Johns]

This it very clear and well stated.
I fully approve of the changes in this form
Thank you for your work on it