That seems like a documentation error here or a function problem.
A read-only item is mostly described so in the docs.
Edit:btw, if you are not sure about how to submit stuff, i personally forget this quite a lot of times myself try this in the terminal command-line:
oprint(renoise.song().tracks[1].postfx_volume)
You then get a list of what is behind it:
>>> oprint(renoise.song().tracks[1].postfx_volume)
class: DeviceParameter
properties:
is_automated
is_automated_observable
name
polarity
show_in_mixer
show_in_mixer_observable
value
value_default
value_max
value_min
value_observable
value_quantum
value_string
value_string_observable
methods:
__STRICT
record_value
reported:
http://www.renoise.c…showtopic=26197