Europa: remove implemented engine definitions from spec mock

Signed-off-by: Solomon Hykes <solomon@dagger.io>

stdlib/europa/dagger/engine/fs.cue

@@ -1,20 +1,33 @@
 package engine
 
+// Read a file from a filesystem tree
 #ReadFile: {
 	_type: "ReadFile"
 
-	input:    #FS
-	path:     string
+	// Filesystem tree holding the file
+	input: #FS
+	// Path of the file to read
+	path: string
+	// Contents of the file
 	contents: string
-	output:   #FS
+	// Output filesystem tree
+	// FIXME: this is a no-op. No output needed.
+	output: #FS
 }
 
+// Write a file to a filesystem tree, creating it if needed
 #WriteFile: {
 	_type: "WriteFile"
 
-	input:    #FS
-	path:     string
+	// Input filesystem tree
+	input: #FS
+	// Path of the file to write
+	path: string
+	// Contents to write
 	contents: string
-	mode:     int
-	output:   #FS
+	// Permissions of the file
+	// FIXME: rename to 'permissions' for consistency
+	mode: int
+	// Output filesystem tree
+	output: #FS
 }