doc: small fixes

from Xavier Chemla
gui stream: display small spectrogram from min of data to max of data

do not start from 0, as 0 has almost no meaning for spectrums
gui acqusition: fix progress bar absolutely never going back

we avoid going back in time for the time estimation, unless the estimation was really wrong.

The logic was flowed preventing from ever going back.
acq: give more margin for CCD in SEM/CCD sync acquisition

some CCD take a long time to start acquiring and a timeout of 1 s is not enough. cf iDus on first frame.
acq stream: improve time estimation of SEM/CCD

do not count the time of the first acquisition, as it's usually much slower than the rest.
driver andorshrk: add note about problem when acquisition is on