FFI: Mention __thiscall attribute in docs.

doc/ext_ffi_semantics.html

@@ -148,7 +148,7 @@ VLA).</li>
 <li>GCC <tt>__attribute__</tt> with the following attributes:
 <tt>aligned</tt>, <tt>packed</tt>, <tt>mode</tt>,
 <tt>vector_size</tt>, <tt>cdecl</tt>, <tt>fastcall</tt>,
-<tt>stdcall</tt>.</li>
+<tt>stdcall</tt>, <tt>thiscall</tt>.</li>
 
 <li>The GCC <tt>__extension__</tt> keyword and the GCC
 <tt>__alignof__</tt> operator.</li>
@@ -160,8 +160,8 @@ function declarations.</li>
 <tt>__int16</tt>, <tt>__int32</tt> and <tt>__int64</tt>.</li>
 
 <li>MSVC <tt>__cdecl</tt>, <tt>__fastcall</tt>, <tt>__stdcall</tt>,
-<tt>__ptr32</tt>, <tt>__ptr64</tt>, <tt>__declspec(align(n))</tt>
-and <tt>#pragma&nbsp;pack</tt>.</li>
+<tt>__thiscall</tt>, <tt>__ptr32</tt>, <tt>__ptr64</tt>,
+<tt>__declspec(align(n))</tt> and <tt>#pragma&nbsp;pack</tt>.</li>
 
 <li>All other GCC/MSVC-specific attributes are ignored.</li>