internal/middleware: add more documentation for requestlog
internal/middleware: add more documentation for requestlog

Fixes b/141160992

Change-Id: I873ff84ccab99b149746aa982207d969faae3b18
Reviewed-on: https://team-review.git.corp.google.com/c/golang/discovery/+/552937
CI-Result: Cloud Build <ea5729d4cf3392b9d382b6ca272c65aa27554f44@system.gserviceaccount.com>
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite