Convert documentation of PKCE functions to our style

[ftsell]

Currently, the documentation of pkce.py is not using the same docstring style as the rest of the codebase.
This should be changed to be more consistent.

pkce.py uses this style:

Parameters
----------
length : int
    Code verifier length. Must verify `43 <= length <= 128`.

whereas other code (e.g. discovery.py) uses this:

:param issuer: The base url of the provider. This url will be appended with `/.well-known/openid-configuration` to retrieve the provider configuration so that must be a valid URL for your provider.
:returns: The well-formed and validated configuration of the given issuer
:raises OpenidProtocolError: When the communication with the provider was not possible or the response was not in an expected format