Documented that headers must fit in buffer

[timmc]

Proposed changes

This clarifies the interaction of proxy_buffer_size and response headers. When the buffer size is too small for the response headers, nginx returns a 502 and logs "upstream sent too big header while reading response header from upstream".

Reference: https://github.com/nginx/nginx/blob/6a67f71a4a78edb662c190af93ac6d3d680e107a/src/http/ngx_http_upstream.c#L2547

Checklist

Before creating a PR, run through this checklist and mark each as complete:

• I have read the contributing guidelines.
• I have signed the F5 Contributor License Agreement (CLA).
• If applicable, I have added tests that prove my fix is effective or that my feature works.
• If applicable, I have checked that any relevant tests pass after adding my changes.
• I have updated any relevant documentation (README.md and/or CHANGELOG.md).

[timmc]

I have hereby read the F5 CLA and agree to its terms

[timmc]

FYI, the PR template has a broken link to the contribution guidelines. I believe it's meant to link to https://github.com/f5/f5-cla/blob/main/CONTRIBUTING.md