Improve docs for Signal.forwardTo

[mgold]

Because a new version of core may be coming out soon, this is a hopefully very simple and uncontroversial change to the docs for Signal.forwardTo. We remove the port keyword, which is wrong and misleading. Then, we define the actions mailbox in a reasonable way rather than just saying that is exists.

[evancz]

Nice, thank you for fixing this! It's a shame the type checker doesn't know about comments :P

[evancz]

Nice, thank you for fixing this! It's a shame the type checker doesn't know about comments :P

[mgold]

Thanks for merging. Although, port actions : Mailbox Action is a valid incoming port declaration, so maybe even automated compilation of docs examples would have missed it!

[mgold]

Thanks for merging. Although, port actions : Mailbox Action is a valid incoming port declaration, so maybe even automated compilation of docs examples would have missed it!