Message Signing Documentation Needs To Be Expanded

[AndrewDelaney]

The message signing documentation needs to be expanded on slightly. Currently there is a single page that contains both the sign_message and verify_message methods, but does not provide much detail on what the methods are used for, particularly in the case of verify_message.

To keep in line with the rest of the documented KDF methods, each method should have its own page where a description of the method should be provided. The current page can still remain to give an overview on message signing and verification and can then referece the actual methods and link to their respective pages.