docs: Add some commas to subdir_done() description

It's subjective but I think that breaking the sentences in this way
makes them easier to follow.

docs/yaml/functions/subdir_done.yaml

@@ -7,9 +7,9 @@ description: |
   will be executed by Meson. This means that all targets defined before
   the call of [[subdir_done]] will be built.
 
-  If the current script was called by `subdir` the execution returns to
+  If the current script was called by `subdir`, the execution returns to
   the calling directory and continues as if the script had reached the
-  end. If the current script is the top level script Meson configures
+  end. If the current script is the top level script, Meson configures
   the project as defined up to this point.
 
 example: |