Doc: Moved FLUX_ISO and CLASS_STAR to isophotal measurement section (which is still named Position.rst, for portability reasons).
Doc: Added ISOAREA description in isophotal shape section.

Doc: Fixed broken ref to Background Modeling section.

Doc: added description of aperture FLUXERR in photometry section.

Doc: Added description on FLUX_PETRO in Photometry section. Re-wrote part of the FLUX_AUTO subsection to better describe "reduced pseudo-radii".

Doc: added Flagging section and flag description tables in Windowed measurement and Weighting sections.

Doc: added a first batch of model-fitting parameters to the list of measurements.