dl: add comments for the dl/example

vlib/dl/dl.v

@@ -5,6 +5,8 @@ pub const (
 	dl_ext  = get_shared_library_extension()
 )
 
+// get_shared_library_extension returns the platform dependent shared library extension
+// i.e. .dll on windows, .so on most unixes, .dylib on macos.
 pub fn get_shared_library_extension() string {
 	mut res := '.so'
 	$if macos {

vlib/dl/example/library.v

@@ -1,10 +1,19 @@
 module library
 
+// add_1 is exported with the C name `add_1`.
+// It can be called by external programs, when the module is compiled
+// as a shared library.
+// It is exported, because the function is declared as public with `pub`.
+// The exported C name is `add_1`, because of the export: tag.
+// (Normally, the exported name is a V mangled version based on the module
+// name followed by __, followed by the fn name, i.e. it would have been
+// `library__add_1`, if not for the export: tag).
 [export: 'add_1']
 pub fn add_1(x int, y int) int {
 	return my_private_function(x + y)
 }
 
+// this function is not exported and will not be visible to external programs.
 fn my_private_function(x int) int {
 	return 1 + x
 }