Document bit manipulation in BandMathX

MuparserX allows bit manipulation (doc), but OTB is getting the image as a float type at present. This prevent the use of such bit operator that throws the error:

Message:     Can't evaluate function/operator "<<": 
Argument 1 of function/operator "im1b1" is of type 'f' whereas type 'i' was expected.
Formula:     im1b1 << 1

EDIT: We need to add documentation as it can be done if one is using the int instruction (see Cédric comments)