DOC: GaussianSource totalflux unit doc is misleading

[pllim]

Given this GaussianSource docstring:

    Parameters
    ----------
    flux : float
        Total flux under the Gaussian curve, in given flux unit.
    center : float
        Central wavelength of the Gaussian curve, in given wavelength unit.
    fwhm : float
        FWHM of the Gaussian curve, in given wavelength unit.
    waveunits, fluxunits : str
        Wavelength and flux units, as accepted by `~pysynphot.units.Units`.
        Defaults are Angstrom and ``flam``.

However, the total flux actually represents the integrated flux, not the monochromatic flux, therefore cannot be represented by the fluxunits. Instead of, say, flam, it is actually erg/s/cm^2.

c/c @vglaidler , not sure if you use Gaussian source in ETC.

xref spacetelescope/synphot_refactor#153