Improve docs for raco ctool --c-mods

pkgs/racket-doc/scribblings/raco/c-mods.scrbl

@@ -24,3 +24,31 @@ Normally, @nonterm{dir} is a relative path, and files are found at run
 time in @nonterm{dir} relative to the executable, but a separate path
 (usually relative) for run time can be specified with
 @DFlag{runtime-access}.
+
+
+Typically, @exec{raco ctool --c-mods} is used with @DPFlag{lib} to
+specify a collection-based module path. For example,
+
+@commandline{raco ctool --c-mods base.c ++lib racket/base}
+
+generates a @filepath{base.c} whose @tt{declare_modules} function
+makes @racketmodname[racket/base] available for use via the
+@tt{scheme_namespace_require} or @tt{scheme_dynamic_require} functions
+within the embedding application.
+
+When a module file is provided to @exec{raco ctool --c-mods}, then
+@tt{declare_modules} declares a module with the symbolic name of the
+module file. For example, 
+
+@commandline{raco ctool --c-mods base.c hello.rkt}
+
+creates a @tt{declare_modules} that defines the module
+@racket['hello], which could be required into the current namespace
+with @racket[(namespace-require ''hello)] or similarly at the C level:
+
+@verbatim[#:indent 2]{
+  p = scheme_make_pair(scheme_intern_symbol("quote"),
+                       scheme_make_pair(scheme_intern_symbol("hello"),
+                                        scheme_make_null()));
+  scheme_namespace_require(p);
+}