Update CLI descriptions in docstrings about pyke

[barentsen]

All these sound like items we should fix.

For 3. (kepstitch): it could be useful for K2 too, in Campaign 9, which was released as two separate half-campaigns (91 and 92).

[mirca]

@gully thanks Gully!

[gully]

13. In kepdiffim, the description says:

Difference imaging of pixels within a target mask

But it does not indicate whether a mask file is required.

[gully]

14. In kepbls CLI the task description is incorrect in argparse:

Remove or replace data outliers from a time series

[gully]

15. In kepmask the CLI indicates that plotfile accepts a default value, but actually requires an argument

: error: argument --plotfile: expected one argument

[mirca]

closing this since most of it has been fixed or reported as separate issues