net/http: document that Basic Auth may require URL encoding
net/http: document that Basic Auth may require URL encoding

Explicitly warn callers that no URL encoding is performed and
that they might need to do it.

Fixes #31577

Change-Id: I52dc3fd2798ba8c3652d4a967b1c5c48eb69f43b
Reviewed-on: https://go-review.googlesource.com/c/go/+/173319
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

akutz/go,josharian/go,akutz/go,CAFxX/go,nawawi/go,akutz/go,nawawi/go,akutz/go,akutz/go,CAFxX/go,CAFxX/go,josharian/go,nawawi/go,josharian/go,akutz/go,cloudflare/go,golang/go,josharian/go,nawawi/go,cloudflare/go,golang/go,josharian/go,CAFxX/go,golang/go,CAFxX/go,cloudflare/go,akutz/go,cloudflare/go,cloudflare/go,golang/go,golang/go,CAFxX/go,cloudflare/go,nawawi/go,nawawi/go,CAFxX/go,cloudflare/go,josharian/go,akutz/go,josharian/go,golang/go,nawawi/go,nawawi/go,CAFxX/go,golang/go,josharian/go,golang/go,cloudflare/go