More sandbox docs

README.md

@@ -65,11 +65,26 @@ the player who pressed the button and keep doing that until they log off. Becaus
 the code does nothing in response to an "act" event, at that point they will go
 into a permanent wait (sleep) state.
 
+### Sandbox
+
+The code runs in a sandboxed Lua environment. Only a limited range of
+functionality is available, and in particular things like loops and functions
+are prohibited.
+
+A range of standard string, table and math library functions are made
+available, as well as the built in minetest vector helpers.
+
+In addition:
+
+* `log(msg)` - logs a v1 debug message
+* `mem` - the person's memory - see under Persistence
+
 ### Persistence
 
-The code runs in a sandboxed Lua environment which persists for as long as the
-person is active. Therefore, 'local' (actually global to the sandboxed
-environment) persist between calls for different events.
+The sandboxed Lua environment persists for as long as the person is active.
+In addition to saving setup time on each call, this allows 'local' (actually
+global to the sandboxed environment) variables persist between calls for
+different events.
 
 Information that needs to persist longer than this (i.e. across deactivations
 or server restarts) can be stored in the person's "memory". The 'mem' table