Add file to documentation

doc/qbk/03_core/4_files.qbk

@@ -16,6 +16,12 @@ underlying filesystem:
 
 [table File Types
 [[Name][Description]]
+[[
+    [link beast.ref.boost__beast__file `file`]
+][
+    `file` is a type alias to one of the following implementations,
+    depending on what is available on the target platform.
+]]
 [[
     [link beast.ref.boost__beast__file_stdio `file_stdio`]
 ][