Clarify Example section and Dl* windows equivalents (ebitengine#274)

Author: TotallyGamerJet

README.md

@@ -35,7 +35,8 @@ except for float arguments and return values.
 
 ## Example
 
-This example only works on macOS and Linux. For a complete example look at [libc](https://github.com/ebitengine/purego/tree/main/examples/libc) which supports Windows and FreeBSD.
+The example below only showcases purego use for macOS and Linux. The other platforms require special handling which can
+be seen in the complete example at [examples/libc](https://github.com/ebitengine/purego/tree/main/examples/libc) which supports Windows and FreeBSD.
 
 ```go
 package main

dlerror.go

@@ -6,6 +6,8 @@
 package purego
 
 // Dlerror represents an error value returned from Dlopen, Dlsym, or Dlclose.
+//
+// This type is not available on Windows as there is no counterpart to it on Windows.
 type Dlerror struct {
 	s string
 }

dlfcn.go

@@ -33,6 +33,10 @@ func init() {
 // A second call to Dlopen with the same path will return the same handle, but the internal
 // reference count for the handle will be incremented. Therefore, all
 // Dlopen calls should be balanced with a Dlclose call.
+//
+// This function is not available on Windows.
+// Use [golang.org/x/sys/windows.LoadLibrary], [golang.org/x/sys/windows.NewLazyDLL], or
+// [golang.org/x/sys/windows.NewLazySystemDLL] for Windows instead.
 func Dlopen(path string, mode int) (uintptr, error) {
 	u := fnDlopen(path, mode)
 	if u == 0 {
@@ -45,6 +49,9 @@ func Dlopen(path string, mode int) (uintptr, error) {
 // It returns the address where that symbol is loaded into memory. If the symbol is not found,
 // in the specified library or any of the libraries that were automatically loaded by Dlopen
 // when that library was loaded, Dlsym returns zero.
+//
+// This function is not available on Windows.
+// Use [golang.org/x/sys/windows.GetProcAddress] for Windows instead.
 func Dlsym(handle uintptr, name string) (uintptr, error) {
 	u := fnDlsym(handle, name)
 	if u == 0 {
@@ -56,6 +63,9 @@ func Dlsym(handle uintptr, name string) (uintptr, error) {
 // Dlclose decrements the reference count on the dynamic library handle.
 // If the reference count drops to zero and no other loaded libraries
 // use symbols in it, then the dynamic library is unloaded.
+//
+// This function is not available on Windows.
+// Use [golang.org/x/sys/windows.FreeLibrary] for Windows instead.
 func Dlclose(handle uintptr) error {
 	if fnDlclose(handle) {
 		return Dlerror{fnDlerror()}