Add note: Semantics defined only for canonical sat/dsat

[dgpv]

Prevent reader confusion, as described in https://github.com/sipa/miniscript/issues/42 where the sematics of "andor" differs from expected by the reader when they consider various executions of the actual script

[dgpv]

Made the reference to the whole "Satisfaction and malleability" section in the note, as not only the "Basic satisfactions" subsection is relevant, but other text there, too