Distinguish data descriptors' permissions in their docstrings

[McSinyx]

In continuation of 83315de and sphinx-contrib/napoleon#22: Currently we have three sorta data descriptors:

1. Read-only properties: decorated by @getter, which is an alias of CPython's @property.
2. Write-only properties: decorated by @setter, which is also an alias of CPython's @property
3. Read-write properties: decorated by @property and @XXX.setter, whose codes are specially handled by Cython

Due to sphinx-doc/sphinx#7448, (3) isn't rendered with the prefix property by Sphinx. Until this gets fix, (3) already stands out from the rest. Nevertheless, my proposal is as follows

1. Use Returns to document all read-only properties
2. Use Parameters to document all write-only properties
3. Use Attributes to document multi-value read-write properties, or when their type needs special attention (at least until Property annotations do not show up in autoclass sphinx-doc/sphinx#7383 gets fix)

[9a24f0]

@McSinyx have you forgotten that we have lecture today?

[McSinyx]

Probably, gentle ping on the issue though!

[McSinyx]

In the process could you please also fix the docs on return value of Decoder.play?

[McSinyx]

Heads up: Sphinx is confused by the Attributes used for read-write properties:

docstring of palace.Source.outer_cone_gains:3: WARNING: duplicate object description of palace.Source.gain, other instance in reference/source, use :noindex: for one of them

I suggest using either Parameters or Returns instead then, but I'm not sure which is better.

Edit: let's go with Return.

[McSinyx]

I've just scanned for Tuple[ in palace.pyx and it seems that this can be closed now.