Use consistent style in docstrings

The following discussion from !5 should be addressed:

• @erhart started a discussion: (+2 comments)

  We have slightly different styles when writing parameter explanations. You start with a capital letter regardless of whether it a sentence and always finish with a dot. (Although below in, e.g., timedomain.py it is lower-case and no dot.) I use lower-case and no dot ;-) Should we agree on one way?