Re: Fix a Oracle-compatible　instr function　in the documentation

I wrote:
> Evidently, they consider that negative beg_index indicates
> the last place where the target substring can *begin*, whereas
> our code thinks it is the last place where the target can *end*.

> After a bit of fooling around with it, I produced code that agrees
> with Oracle as far as I can tell, but it wouldn't be a bad idea
> for someone else to test it some more.

I spent some more time comparing this version's behavior with
rextester's Oracle instance, and couldn't find any other
discrepancies, so I pushed it.

            regards, tom lane