doc: fill out net/http.ErrSchemeMismatch note

doc: fill out net/http.ErrSchemeMismatch note

For #44855

Change-Id: I517394b227ea948ed3e1f9ffdaab2bb2676863c8
Reviewed-on: https://go-review.googlesource.com/c/go/+/496795
TryBot-Bypass: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>