Custom Name Resolution page incorrectly lists DNS as the default resolver

**Issue description**
The [Custom Name Resolution page](https://grpc.io/docs/guides/custom-name-resolution/) says "By default, DNS is assumed." but the [actual code for the Go client](https://github.com/grpc/grpc-go/blob/master/resolver/resolver.go#L40) 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.