Custom Name Resolution page incorrectly lists DNS as the default resolver

[xinxin-stripe]

Issue description
The Custom Name Resolution page says "By default, DNS is assumed." but the actual code for the Go client sets "passthrough" as the default scheme. I haven't checked what happens in other languages.

Steps to reproduce the issue
See the issue description.

Expected behavior
The page should say the resolver is "Passthrough" or "Language-dependent" for the grpc.io:50051 example and not say "By default, DNS is assumed."

It can also have a row explicitly for "Passthrough" using passthrough:///grpc.io:50051.

For example:

Resolver	Example	Notes
Language-dependent	grpc.io:50051	In Go, Passthrough is used by default. In Java, ....
Passthrough	passthrough:///grpc.io:50051	Unlike the DNS resolver, this creates a single sub-connection instead of pre-resolving the DNS and making a separate sub-connection for each IP address.

Screenshots
This is what the page currently looks like:

Desktop (if relevant):
N/A

Mobile (if relevant):
N/A

Additional info
It took me and my coworkers a long time and digging into the gRPC client source code to debug why our gRPC client wasn't working as expected.