Update messages.md to include missing parameter

[joearcher]

Add undocumented optional visible_to_clients parameter to message creation endpoint documentation

[gavrichards]

Hey @joearcher, interesting PR. Do you know if this parameter works for updating a message too? And possibly also for other entities like to-do lists?

[joearcher]

Hey @gavrichards currently untested, I just built an integration that allows us to message all of our clients in bulk, so it only uses the message creation endpoint at present.

[gavrichards]

@joearcher gotcha, thanks!

[gavrichards]

Sorry, hidden my earlier comments as I realised I'd misremembered, and in fact updating existing messages and to-do lists to be visible to clients is already possible. The functionality I'm looking for is to be able to set the client visibility status for items in a project template.