(ice-9 documentation) `find-documentation' to be exported

[thi]

currently, there are two definitions exported from (ice-9
documentation): `documentation-files', a search-list used by
`find-documentation'; and `object-documentation', a proc that returns
the docstring of an object.

because `object-documentation' can only work on in-core objects, there
is no way to look at documentation for procs not yet (or maybe never to
be) loaded.  having a search-list but no way to do the search seems sort
of half-baked...

we can fix this by exporting `find-documentation', an already provided
proc that accepts a symbol and does the search w/o requiring residency.

this enables people to do stuff like:

(use-modules (ice-9 documentation))
(or (member "my-docs.txt" documentation-files)
    (set! documentation-files (cons "my-docs.txt" documentation-files)))
(find-documentation 'my-proc)

see also the related guile-sources post of a simple scheme-only
docstring snarfer with output compatible with this usage.

if nobody objects by eow, i will export `find-documentation' and do a
small writeup for (ice-9 documentation) as well.  related question: how
stable is the current guile-procedures.txt format?  (i.e., can this be
documented at this time?)

thi