Inconsistent doc around midi message helper methods

[pissalou]

file.md and midi-1-out.md mention helper methods like note_on and note_off to be defined in a structure called libremidi::message. While these methods are declared in message.hpp file, they belong to another structure called channel_events.

[jcelerier]

thanks for the report ! I'll update it. Indeed I moved them out of the class and forgot to update the docs.