DOC/MINOR: update request-capture documentation to clarify header handling

[odoucet]

This pull request updates the documentation to clarify and correct how to capture HTTP headers that may contain commas (such as User-Agent) in HAProxy annotations. It emphasizes the importance of using req.fhdr(header-name) instead of hdr(header-name) to avoid issues with truncated or malformed log entries, and updates all relevant examples accordingly.

This is a followup of #729

Documentation improvements:

• Added a warning in documentation/doc.yaml explaining that hdr(header-name) splits values on commas, which can cause problems when capturing headers like User-Agent, and instructing users to use req.fhdr(header-name) instead.
• Updated all header capture examples in documentation/doc.yaml to use req.fhdr(user-agent) instead of hdr(User-Agent).
• Clarified the list of possible values for request capture expressions to include both hdr(header-name) and req.fhdr(header-name) for headers with commas.

[hdurand0710]

LGTM