BUG: parameters should be documented in docstring / help

e.g. this is inadequate, the parameter descriptions should be exposed to the users (after all that is what they are after). I haven't checked, but this may be wider spread than just SIA.

```
>>> import pyvo
>>> irsa_SIA = pyvo.dal.SIA2Service('https://irsa.ipac.caltech.edu/SIA')
>>> irsa_SIA.search?
Signature:
irsa_SIA.search(
pos=None,
band=None,
time=None,
pol=None,
field_of_view=None,
spatial_resolution=None,
spectral_resolving_power=None,
exptime=None,
timeres=None,
publisher_did=None,
facility=None,
collection=None,
instrument=None,
data_type=None,
calib_level=None,
target_name=None,
res_format=None,
maxrec=None,
session=None,
**kwargs,
)
Docstring:
Performs a SIA2 search against a SIA2 service

See Also
--------
pyvo.dal.sia2.SIA2Query
File: ~/munka/devel/pyvo/pyvo/dal/sia2.py
Type: method
```

Start in pyvo/dal/sia2.py at SIA2Service.search and inspect how its parameters are currently documented. Compare related DAL service methods to determine whether the issue affects more than SIA, then ensure parameter descriptions appear in the generated docstring or interactive help for the affected methods.

Written by the indexing model from the issue text.