correctly document ilike

[Mike Bayer]

ilike() seems to have generated "lower(x) = lower(y)" for years by default since ilike is not standard SQL, however this is not documented at all in its docstring, it claims to only generate ILIKE. Similarly, add ilike() with a note that this generates "ILIKE or lower(x) = lower(y)" in the ORM (SQL?) tutorial(s).

[Mike Bayer]

• correctly document LIKE / ILIKE, fixes #3890

Change-Id: Ie59e61f53d7c59a4777ab9e6e75a43c71d67523b

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