add note about keywords and label names

Author: JPryce-Aklundh

modules/ROOT/pages/syntax/keywords.adoc

@@ -54,6 +54,9 @@ RETURN `true` AS x
 1+d|Rows: 1
 |===
 
+[NOTE]
+The keywords `NOT`, `NULL`, `TYPED`, `NORMALIZED`, `NFC`, `NFD`, `NFKC`, and `NFKD` are not allowed as label names after the keyword `IS` unless they are quoted by backticks.
+For example, `CREATE (n IS NOT)` is invalid, but `CREATE (n IS ++`NOT`++)` is allowed.
 
 == List of keywords