meta: update luacheckrc

.luacheckrc

@@ -1,10 +1,14 @@
 -- Don't report unused self arguments of methods.
 self = false
 
+-- Rerun tests only if their modification time changed.
+cache = true
+
 ignore = {
     "631", -- max_line_length
 }
 
+-- Global objects defined by the C code
 globals = { "vim" }
 
 -- vim: ft=lua tw=80