Clarify Pipeline.windowBy() documentation about pond

[montegoode]

For what it's worth I spruced up the doc string and also changed the arg name to be a bit more friendly:

    def window_by(self, window_or_duration=None):
        """
        Set the window, returning a new Pipeline. The argument here
        is either a string or an object with string attrs type and
        duration. The arg can be either a window or a duration.

        If no arg is supplied or set to None, the window_type is set
        to 'global' and there is no duration.

        Window *window_or_duration* may be:

        * A fixed interval: "fixed"
        * A calendar interval: "daily," "monthly" or "yearly"

        Duration is of the form:

        * "30s" or "1d" etc - supports seconds (s), minutes (m), hours (h),
          days (d). When duration is passed as the arg, window_type is
          set to 'fixed'.

        Parameters
        ----------
        window_or_duration : string, Capsule
            See above.

        Returns
        -------
        Pipeline
            The Pipeline.
        """

[pjm17971]

In 0.7 branch.