bug#4727: 23.1; `multi-isearch-(files|buffers)(-regexp)'

[Drew Adams]

> However, we could convert relative file names to internal absolute
> file names before starting multi-file Isearch.  So you will be able
> to specify file names relative to the default directory where
> multi-file Isearch was started.

That was what I was thinking.

> > 3. Similarly, for `multi-isearch-buffers(-regexp)':
> >
> > a. The doc strings need to say explicitly that the BUFFERS must be
> > live buffers, not their names.
> >
> > b. Why should the BUFFERS need to be buffers - why not also allow
> > buffer names?
> >
> > The code is unnecessarily restrictive/brittle.
> 
> Similarly, we could convert buffer names to internal live buffers
> before starting multi-buffer Isearch.

Why not? That's pretty standard. Let the function do the work of calling
`get-buffer'.