BF(DOC): list values of 'string' choices in Parameter's

with such changes ```choices``` provided to a Parameter would be included into a docstring

```
$> PYTHONPATH=$PWD python -c 'from mvpa2.clfs.gda import LDA; print LDA.__init__.__doc__'                   
Initialize a GDA classifier.
 

Parameters
----------
prior
  How to compute prior distribution. [Choices: 'laplacian_smoothing',
  'uniform', 'ratio'] (Default: 'laplacian_smoothing')
allow_pinv
...
```