Docstrings for parameters

Dieses Issue hat noch niemand übernommen.

Given a situation like the following:

``` python
# foo driver,pseudo code
self.add_parameter('foo',
label='foo',
set_cmd='bar',
set_parser=self._set_input_config,
)

def _set_input_config(self, s):
if s in ['baz']:
self.foo.set_validator(self._VOLT_ENUM)
self._set_units('V')
else:
self.foo.set_validator(self._CURR_ENUM)
self._set_units('A')

```

The question is then how to document the parameter foo?

The same would be if one adds a foo{bar} parameter depending on some response of another parameter right ?

Two solutions not mutally exclusive:
- docstring that describe "dynamically" the parameters (may be not trivial to implement)
- describe all the possible behaviors of the parameter.

I can't come up with a docstring example for the latter case though.

Also we should just decide on what's the best, not what's doable at the moment with the current architecture.

@alexcjohnson @MerlinSmiles @Rubenknex @AdriaanRol write your opinions (tagged because you were part of the discussion in #139 .

Beginne damit, das Parameterbeispiel in diesem Issue und die frühere Diskussion in #139 zu prüfen. Ermittle, wie dynamisch geänderte Parameter in docstrings dargestellt werden sollten, einschließlich abhängiger foo{bar}-Parameter, und lege einen abgestimmten Dokumentationsansatz fest, der alle möglichen Verhaltensweisen abdeckt.

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