Add some documentation.

theories/si_logic/bi.v

@@ -5,6 +5,9 @@ Import siProp_primitive.
 (** BI instances for [siProp], and re-stating the remaining primitive laws in
 terms of the BI interface.  This file does *not* unseal. *)
 
+(** We pick [*] and [-*] to coincide with [∧] and [→], respectively. This seems
+to be the most reasonable choice to fit a "pure" higher-order logic into the
+proofmode's BI framework. *)
 Definition siProp_emp : siProp := siProp_pure True.
 Definition siProp_sep : siProp → siProp → siProp := siProp_and.
 Definition siProp_wand : siProp → siProp → siProp := siProp_impl.

theories/si_logic/siprop.v

 From iris.algebra Require Export ofe.
 From iris.bi Require Import notation.
 
+(** The type [siProp] defines "plain" step-indexed propositions, on which we
+define the usual connectives of higher-order logic, and prove that these satisfy
+the usual laws of higher-order logic. *)
 Record siProp := SiProp {
   siProp_holds :> nat → Prop;
   siProp_closed n1 n2 : siProp_holds n1 → n2 ≤ n1 → siProp_holds n2