Clarify the operator that icontains uses on Postgresql

[Artyem Klimenko]

​https://docs.djangoproject.com/en/1.8/ref/models/querysets/#icontains

SQL equivalent:
SELECT ... WHERE headline ILIKE '%Lennon%';

ilike exist in PostgreSQL, but not used, and it is not obvious.

q = NetworkObject.objects.filter(string_view__icontains='test').only('id')
print(q.query)
SELECT "core_networkobject"."id" FROM "core_networkobject" WHERE UPPER("core_networkobject"."string_view"::text) LIKE UPPER(%test%)

I understand that you can not correct the behavior icontains, but you can clarify this issue in the documentation.

And maybe add to the documentation this lookup.

from django.db import models
from django.db.models.lookups import IContains

class PostgreILike(IContains):
    lookup_name = 'ilike'

    def as_postgresql(self, compiler, connection):
        lhs, lhs_params = self.process_lhs(compiler, connection)
        rhs, rhs_params = self.process_rhs(compiler, connection)
        params = lhs_params + rhs_params
        return '%s ILIKE %s' % (lhs, rhs), params

models.CharField.register_lookup(PostgreILike)
models.TextField.register_lookup(PostgreILike)

P.S. Sorry for my poor English.

[Tim Graham]

Duplicate of #20775