Translate section 'Connexion n - m (demux / mux)

In the course of this translation, I noticed two issues:

* the role of components A and B is ambiguous:
  - which one is the input?
  - which one is the output?

  The formulae are unambiguous, but the text should be too. As a sign of this
  confusion, the index 'n' used for component A in the text is used for the
  properties of component B in the first formula.

* the formulae for the multiplexer and the demultiplexer were reversed:
  - mux (not demux) is one resulting from f ( several )
  - demux (not mux) is several resulting from f ( one )

Reference:
http://en.wikipedia.org/wiki/Multiplexer