litrpc: update MacaroonPermissions comments

The proto comments are updated to reflect the special case behaviour of
the entity-action pairs.

Author: ellemouton

litrpc/lit-sessions.pb.go

@@ -36,10 +36,18 @@ message AddSessionRequest {
 }
 
 message MacaroonPermission {
-    // The entity a permission grants access to.
+    // The entity a permission grants access to. If a entity is set to the
+    // "uri" keyword then the action entry should be one of the special cases
+    // described in the comment for action.
     string entity = 1;
 
-    // The action that is granted.
+    // The action that is granted. If entity is set to "uri", then action must
+    // be set to either:
+    //  - a particular URI to which access should be granted.
+    //  - a URI regex, in which case access will be granted to each URI that
+    //    matches the regex.
+    //  - the "***readonly***" keyword. This will result in the access being
+    //    granted to all read-only endpoints.
     string action = 2;
 }