Document the right use of named parameter substitution in N1QL

Please document the right usage for named parameter substitution for N1QL queries. The following query throws a syntax error regarding the use of named parameter substitution -

UPDATE myBucket USE KEYS["myDocId"] SET $myCounter = $myCounter + 1; 

error is as follows -

[

  {

    "code": 3000,

    "msg": "syntax error - at $myCounter",

    "query_from_user": "UPDATE destination USE KEYS[\"myDocId\"] SET $myCounter = $myCounter + 1;"

  }

] 

I recollect that there was page in documentation describing the clauses (such as WHERE, FROM etc.) only after which the named parameter substitution was permissible. But I'm unable to find it now.