Minor tweaks to time-stamp documentation strings

Minor tweaks to time-stamp documentation strings

* time-stamp.el (time-stamp): Reformat the explanation of the
variables that affect time-stamp, for easier reading.  In particular,
wrap the documentation to 75 characters, so that it displays neatly as
a before-save-hook customization option.

* time-stamp-tests.el (with-time-stamp-test-env): Use imperative voice,
per checkdoc.