finagle-docs: Add Http Headers to protocol section
finagle-docs: Add Http Headers to protocol section

Problem / Solution

Finagle uses http headers transmit request/response metadata, but those headers
are not documented anywhere. This adds header documentation to the Protocols
section of Finagle docs.

RB_ID=915340

koshelev/finagle,luciferous/finagle,twitter/finagle,mkhq/finagle,koshelev/finagle,mkhq/finagle,luciferous/finagle,mkhq/finagle,mkhq/finagle,koshelev/finagle,twitter/finagle,twitter/finagle,luciferous/finagle,luciferous/finagle,twitter/finagle,mkhq/finagle,luciferous/finagle,koshelev/finagle,luciferous/finagle,koshelev/finagle,twitter/finagle,mkhq/finagle,koshelev/finagle