source/docker.py: Document os/architecture as since 2.0.1

Technically these could not be used in 2.0.0 because validation would
reject these keys, so document this coherently.
diff --git a/src/buildstream_plugins/sources/docker.py b/src/buildstream_plugins/sources/docker.py
index 32768ef..45628fa 100644
--- a/src/buildstream_plugins/sources/docker.py
+++ b/src/buildstream_plugins/sources/docker.py
@@ -47,8 +47,11 @@
    # will choose which image to use based on these settings. Default values
    # are chosen based on the output of `uname -m` and `uname -s`, but you
    # can override them.
-   #architecture: arm64
-   #os: linux
+   #
+   # architecture: arm64
+   # os: linux
+   #
+   # **Since**: 2.0.1
 
 Note that Docker images may contain device nodes. BuildStream elements cannot
 contain device nodes so those will be dropped. Any regular files in the /dev