[doc] FT_Open_Args: Clarify the ownership of `pathname'.

Don't expect the function to deallocate the buffer.  This may be obvious
in C, but it is not so obvious in FFI bindings for FreeType 2.
diff --git a/include/freetype/freetype.h b/include/freetype/freetype.h
index a6bb667..5eb613b 100644
--- a/include/freetype/freetype.h
+++ b/include/freetype/freetype.h
@@ -2078,7 +2078,8 @@
    *     The size in bytes of the file in memory.
    *
    *   pathname ::
-   *     A pointer to an 8-bit file pathname.
+   *     A pointer to an 8-bit file pathname.  The pointer is not owned by
+   *     FreeType.
    *
    *   stream ::
    *     A handle to a source stream object.