Document the .id() method.

README.rst

@@ -35,6 +35,7 @@ SYNOPSIS
   BYTES <obj>.size()
   TIME <obj>.mtime()
   DURATION <obj>.next_check()
+  BLOB <obj>.id()
 
   # VMOD version
   STRING file.version()
@@ -457,7 +458,19 @@ Example::
 BLOB xreader.id()
 -----------------
 
-XXX ...
+Returns a unique, opaque identifier for the state of the file as
+determined when it was most recently checked. The ID changes if and
+only if the file was found to have changed.
+
+Example::
+
+  import blob;
+
+  set resp.http.ETag = blob.encode(BASE64, blob=rdr.id());
+
+The contents of the BLOB returned by ``.id()`` are intentionally not
+documented, and should not be relied on to extract information about
+the file.
 
 .. _file.version():
 

src/vmod_file.vcc

@@ -31,6 +31,7 @@ SYNOPSIS
   BYTES <obj>.size()
   TIME <obj>.mtime()
   DURATION <obj>.next_check()
+  BLOB <obj>.id()
 
   # VMOD version
   STRING file.version()
@@ -413,7 +414,19 @@ Example::
 
 $Method BLOB .id()
 
-XXX ...
+Returns a unique, opaque identifier for the state of the file as
+determined when it was most recently checked. The ID changes if and
+only if the file was found to have changed.
+
+Example::
+
+  import blob;
+
+  set resp.http.ETag = blob.encode(BASE64, blob=rdr.id());
+
+The contents of the BLOB returned by ``.id()`` are intentionally not
+documented, and should not be relied on to extract information about
+the file.
 
 $Function STRING version()