Attempt to improve comments for HttpResponseHeaders methods.
Attempt to improve comments for HttpResponseHeaders methods.

Remove "remove this" comment from GetNormalizedHeader and add an explanation
to EnumerateHeader to explain the transformation it does and when it might
be better to call GetNormalizedHeader instead.

Review URL: https://codereview.chromium.org/1728673003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#377686}

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