Add <dlfcn.h> to the libdl documentation.

Currently only <android/dlext.h> is there.

Change-Id: I733205983d47c83543e6277391e2de5a6df24e25
diff --git a/libc/include/dlfcn.h b/libc/include/dlfcn.h
index 67759b8..81045fd 100644
--- a/libc/include/dlfcn.h
+++ b/libc/include/dlfcn.h
@@ -32,6 +32,17 @@
 
 #include <stdint.h>
 
+/**
+ * @addtogroup libdl Dynamic Linker
+ * @{
+ */
+
+/**
+ * \file
+ * Standard dynamic library support.
+ * See also the Android-specific functionality in `<android/dlext.h>`.
+ */
+
 __BEGIN_DECLS
 
 /**
@@ -52,6 +63,8 @@
  * [dlopen(3)](https://man7.org/linux/man-pages/man3/dlopen.3.html)
  * loads the given shared library.
  *
+ * See also android_dlopen_ext().
+ *
  * Returns a pointer to an opaque handle for use with other <dlfcn.h> functions
  * on success, and returns NULL on failure, in which case dlerror() can be used
  * to retrieve the specific error.
@@ -191,3 +204,5 @@
 #endif
 
 __END_DECLS
+
+/** @} */