docs: returned header size reflects HTTP/1-style format

[vszakats]

Ref: #21889

[Copilot]

Pull request overview

Updates the CURLINFO_HEADER_SIZE documentation to clarify that the reported header size corresponds to libcurl’s HTTP/1-style textual header representation, which is particularly relevant when using HTTP/2+ where on-the-wire header bytes differ due to framing/compression.

Changes:

• Clarify that headers are measured in an HTTP/1-style textual format rather than raw on-the-wire bytes.
• Add a note that HTTP/2+ wire/TLS-layer byte counts differ from the reported value.

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.