Document current-thread-data

docs/api/srfi/18.md

@@ -6,6 +6,8 @@ See the [Multithreading support SRFI documentation](http://srfi.schemers.org/srf
 
 - [`thread?`](#thread)
 - [`make-thread`](#make-thread)
+- [`current-thread`](#current-thread)
+- [`current-thread-data`](#current-thread-data)
 - [`thread-name`](#thread-name)
 - [`thread-specific`](#thread-specific)
 - [`thread-specific-set!`](#thread-specific-set)
@@ -40,6 +42,18 @@ Determine if the given object is a thread object.
 
 Create a new thread object.
 
+# current-thread
+
+    (current-thread)
+
+Returns the current thread.
+
+# current-thread-data
+
+    (current-thread-data)
+
+Returns the `gc_thread_data` instance for the current thread, wrapped in an opaque object. This is useful when using the FFI as many functions in the Cyclone runtime require the `gc_thread_data` data argument.
+
 # thread-name
 
     (thread-name t) (vector-ref t 3))