bug#10326: Need to document %load-path, %load-compiled-path, and add %search-load-compiled-path and load-compiled-from-path to API

[Ian Hulin <ian@hulin.org.uk>]

Section 6.17.6 of the manual could do with a sections for %load-path
and %load-compiled-path.

e.g.
— Variable: %load-path

    The list of directories which will currently be searched by
load-from-path, primitive-load-path and %search-load-path to locate a
scheme file.

- Variable: %load-compiled-path

    The list of directories which will currently be searched by
load-compiled-from-path and %search-load-compiled-path to locate a
compiled scheme file.


Also, Guile really needs to provide a %search-load-compiled-path and a
load-compiled-from-path, it would have been useful in LilyPond code, and
Manual Section 4.7 (discussing site locations says)
"As with Scheme files, Guile searches a path to find compiled .go
files, the %load-compiled-path. By default, this path has two entries:
a path for Guile's files, and a path for site packages. You should
install your .go files into the latter. Currently there is no
procedure to get at this path, which is probably a bug."


— Scheme Procedure: %search-load-compiled-path filename
— C Function: scm_sys_search_load_compiled_path (filename)

    Search %load-compiled-path for the file named filename, which must
be readable by the current user. If filename is found in the list of
paths to search or is an absolute pathname, return its full pathname.
Otherwise, return #f.

— Scheme Procedure: load-compiled-from-path filename

    Similar to load-compiled, but searches for filename in the
compiled load paths.