Attribute's "format" property description is incomplete

[mliszcz]

The doc describes the "format" property as a set of c++ stream manipulators separated with colons:
https://tango-controls.readthedocs.io/en/latest/development/advanced/reference.html#the-format-attribute-parameter

But it seems that a printf-like (lib C) format is also a valid format string in tango (actually %6.2f is default format value for scalars):

cppapi/server/tango_const.h.in
309:const char* const FormatNotSpec_FL         = "%6.2f";

ATK supports both manipulator and printf-like syntax:
https://github.com/tango-controls/atk/blob/8db92a434b14e6b8c2f65afbf353b43eea3dc38e/widget/src/main/java/fr/esrf/tangoatk/widget/attribute/SimpleScalarViewer.java#L302-L308