Add doc to lsm_btree.hrl

include/lsm_btree.hrl

@@ -6,6 +6,15 @@
 %%
 -define(BTREE_ASYNC_CHUNK_SIZE, 100).
 
+%%
+%% The btree_range structure is a bit assymetric, here is why:
+%%
+%% from_key=<<>> is "less than" any other key, hence we don't need to
+%% handle from_key=undefined to support an open-ended start of the
+%% interval. For to_key, we cannot (statically) construct a key
+%% which is > any possible key, hence we need to allow to_key=undefined
+%% as a token of an interval that has no upper limit.
+%%
 -record(btree_range, { from_key = <<>>       :: binary(),
                        from_inclusive = true :: boolean(),
                        to_key                :: binary() | undefined,