XML Message Interface Documention Methods

Can somebody tell me a good way to document an xml message interface from scratch.  That is,  I have the following message format (something that is similar in nature):

<SomeMsgRequest>
  <RequiredElement/>
  <OptionalElement/>
</SomeMsgRequest>

And need a good way document the message and what tags are required and which are optional.
Are there any tools out there that can help with this...possibly using an xsd or schema?

Any information is very much appreciated.

Thanks!