N1QL build index expressions should mention array of identifiers

Not really a bug but want to make sure this is seen. For mad hatter we have a new build index function that allows one to pass in strings, identifiers and arrays of strings for the index names. We should make special note that arrays of identifiers are not allowed. This could cause some confusion if a developer is trying to write a script that uses the new build index syntax.

Allowed:

BUILD index on default('ix1', 'ix2', 'ix3')

BUILD index on default(ix1, ix2, ix3)

BUILD index on default(['ix1', 'ix2', 'ix3'])

any mix of the previous

Not allowed:

BUILD index on default([ix1, ix2, ix3]) --> array of identifiers (not strings)