varlink: comment that "more" flag IDL comment is API

varlink: comment that "more" flag IDL comment is API

External tools that use the systemd varlink ecosystem require
to know if a specific varlink method supports/requires the
"more" flag from the IDL. This is tracked upstream in
https://github.com/varlink/varlink.github.io/issues/26

As an intermediate step systemd adds the (very nice) comments
```
 # [Requires 'more' flag]
or
 # [Supports 'more' flag]
```
to the various methods.

This commit extends the comment around the code that adds the
comment to clarify that this should be considered API and that
the comment should not be changed as external tools (like e.g.
the varlink-http-bridge) rely on it.