Update compatibility.md

Add a note about function identity

docs/_pages/compatibility.md

@@ -127,4 +127,5 @@ We have a few behavior deviations from Lua 5.x that come from either a different
 * Tail calls are not supported to simplify implementation, make debugging/stack traces more predictable and allow deep validation of caller identity for security
 * Order of table assignment in table literals follows program order in mixed tables (Lua 5.x assigns array elements first in some cases)
 * Equality comparisons call `__eq` metamethod even when objects are rawequal (which matches other metamethods like `<=` and facilitates NaN checking)
+* `function()` expressions may reuse a previosly created closure in certain scenarios (when all upvalues captured are the same) for efficiency, which changes object identity but doesn't change call semantics -- this is different from Lua 5.1 but similar to Lua 5.2/5.3
 * `os.time` returns UTC timestamp when called with a table for consistency