document canvas_realizedollar()

This makes sure that the object text on the canvas reflects what's been
set with set_args(). Previously the object text was never updated, which
caused a lot of confusion because of the mismatch between visual
representation and internal state.

Note that the object text is invisible for graphical objects, so there
are no visible changes for those, but the object text gets updated in
either case.

self:get_args() returns an object's arguments in a Lua table. float
atoms are returned as numbers, other types of atoms as a string using
their print representation in Pd syntax (including escapes of special
symbols according to Pd's quoting rules).

Note that there are some special kinds of Pd atoms such as pointers
which don't have a serializable string representation. But as long as
all arguments are float and symbol atoms, the resulting table can be
passed to self:set_table() to re-create the same object arguments.