net/http: flesh out Request.SetBasicAuth docs a bit
net/http: flesh out Request.SetBasicAuth docs a bit

Clarify that username can't contain a colon.
See https://go.dev/play/p/aM25UHC6n98

Change-Id: I342575107104cbcd28b1d3442c474adf93b7f03c
Reviewed-on: https://go-review.googlesource.com/c/go/+/394115
Trust: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

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