Clarify nature of calling convention

[Tetralux]

Also rephrase following sentence about slices, dynarrays and maps not being special types.

[Mecso2]

you could also mention how multiple return values work:
the last return value is returned in the traditional way
for the rest the the caller creates space on its stack and passes a pointer as an extra argument
there's one pointer for each return value (except for the last)
these extra arguments come right before the context pointer