Remove and warn about STRICT in functions

[steve-chavez]

Related to PostgREST/postgrest#1294.

[steve-chavez]

The definition of STRICT:

STRICT indicates that the function always returns null whenever any of its arguments are null. If this parameter is specified, the function is not executed when there are null arguments; instead a null result is assumed automatically.

Personally I never use it. I think I'll just remove the keyword, if I go into details of the performance implications the note could get too long.

[steve-chavez]

I think this recommendation should go in our schema structure now.