Msg Shard documentation should be updated to reflect its intention for only String rvalues

[Yttruire]

Describe the bug

Current documentation suggests that "The message to display can be passed to this shard either as a string or as a variable holding a value. In case a variable is passed the shard displays the variable's value.".

This is not intentional, and Msg should only accept rvalues and not variables nor arguments. The documentation should be updated to reflect accordingly once the behaviour of the Msg shard has been rectified