docs: added --platform=local example

Signed-off-by: David Karlsson <david.karlsson@docker.com>

docs/reference/buildx_build.md

@@ -414,8 +414,13 @@ The `registry` exporter is a shortcut for `type=image,push=true`.
 
 Set the target platform for the build. All `FROM` commands inside the Dockerfile
 without their own `--platform` flag will pull base images for this platform and
-this value will also be the platform of the resulting image. The default value
-will be the current platform of the buildkit daemon.
+this value will also be the platform of the resulting image.
+
+The default value is the platform of the BuildKit daemon where the build runs.
+The value takes the form of `os/arch` or `os/arch/variant`. For example,
+`linux/amd64` or `linux/arm/v7`. Additionally, the `--platform` flag also supports
+a special `local` value, which tells BuildKit to use the platform of the BuildKit
+client that invokes the build.
 
 When using `docker-container` driver with `buildx`, this flag can accept multiple
 values as an input separated by a comma. With multiple values the result will be