bug#14843: 24.3.50; `line-move', `line-move-visual' need doc strings

[Drew Adams]

> > These functions, especially `line-move', are used all over the place.
> > They need doc strings, with complete descriptions of their parameters.
> 
> They are internal functions.  Under what circumstances did you need to
> use them directly?  Why wasn't next/previous-line enough?

Read what I wrote: they "are used all over the place".  In the Emacs code
alone, not to mention in 3rd-party code.  They are not "internal" to
simple.el.  They are not internal to anything.  Reality.