document legacy OGS subsystem support

documentation/dev_man.md

@@ -18,3 +18,20 @@
 | int input_mode | Responsible of keeping track how current input should be handled. Not only does  the readline handler depend on it, its also used to determine what types of inputs all legal (e.g. swapping to another window). Takes up on of the values of the INPUT_\* macros.
 | int window_change | Bit mask type of the CH_\* macros. Keeps track of the windows to be refresed on the next run of display(). Could be better utalized.
 
+# Notes
+CScope used to support something
+refered to as "OGS book and subsystem names".
+I have literally no clue what OGS is in reference to,
+it may or may not be the Quake II engine.
+Based on the code,
+the idea is that folders ending `.ss/` are "subsystems",
+and folders below subsystems are "books".
+CScope would display the subsystem and book names
+in its table view as any other field,
+given that the `ogs` global was set.
+So how do you set the ogs global?
+You don't,
+it was never documented,
+and there was never a way to set it at run time.
+I assume one project wanted it
+and sneaked in the patch.