Add an example for go_download_sdk.sdks (bazelbuild#3139)

Fixes bazelbuild#3136.

go/toolchains.rst

@@ -296,7 +296,20 @@ This downloads a Go SDK for use in toolchains.
         name = "go_sdk",
         goos = "linux",
         goarch = "amd64",
-        version = "1.12.5",
+        version = "1.18.1",
+        sdks = {
+            # NOTE: In most cases the whole sdks attribute is not needed.
+            # There are 2 "common" reasons you might want it:
+            #
+            # 1. You need to use an modified GO SDK, or an unsupported version
+            #    (for example, a beta or release candidate)
+            #
+            # 2. You want to avoid the dependency on the index file for the
+            #    SHA-256 checksums. In this case, You can get the expected
+            #    filenames and checksums from https://go.dev/dl/
+            "linux_amd64": ("go1.18.1.linux-amd64.tar.gz", "b3b815f47ababac13810fc6021eb73d65478e0b2db4b09d348eefad9581a2334"),
+            "darwin_amd64": ("go1.18.1.darwin-amd64.tar.gz", "3703e9a0db1000f18c0c7b524f3d378aac71219b4715a6a4c5683eb639f41a4d"),
+        },
     )
 
     go_rules_dependencies()