http: improve Get documentation
http: improve Get documentation

The existing documentation is unclear about header keys formatting.
The clarifying sentence is added to Get function to emphasis that
keys have to be stored in canonical format to have Get returining
non empty value.

Fixes #53140

Change-Id: Icd0955bcbb6676cec028fe37042aed5846e13ed1
Reviewed-on: https://go-review.googlesource.com/c/go/+/417975
Run-TryBot: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>
Auto-Submit: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>

nawawi/go,nawawi/go,golang/go,golang/go,nawawi/go,nawawi/go,golang/go,nawawi/go,nawawi/go,golang/go,nawawi/go,nawawi/go,golang/go,golang/go,golang/go,golang/go