DOC: add detail about StateSpace.dcgain
DOC: add detail about StateSpace.dcgain

Thanks to Rory Yorke (@roryyorke) and Richard M. Murray (@murrayrm)
for comments and suggesting text.

based on discussion at
https://github.com/python-control/python-control/commit/ee75d9aab7f36f565fb3ddb3c9699be6c2084173

python-control/python-control,murrayrm/python-control,roryyorke/python-control