add a warning for callback and array with less than 2 items

[alexislefebvre]

Fixes #4760

• Mention that usort doesn't call the callback if the array has only one element? #4760

I can't test it right now because of an issue with git:

• Fatal error from git when setting up a local development environment doc-base#247

Here is the result:

[nielsdos]

Small suggestion to split it up in 2 sentences, and a small correction (error->warning):

Suggested change

	The callback is not called if the input has less than 2 items,
	an error (e.g. accessing an invalid key of an array) or an exception
	will not be reported, unless you test it with 2 items or more.
	The callback is not called if the input has less than 2 items.
	A warning (e.g. accessing an invalid key of an array) or an exception
	will not be reported, unless you test it with 2 items or more.

What do you think?

[alexislefebvre]

I'm trying to explicitly explain that an error will go unnoticed, see the example here:

• Mention that usort doesn't call the callback if the array has only one element? #4760

Or this one: https://3v4l.org/k0bHmC#vnull

[nielsdos]

I see. The reason I suggested warning is because invalid array keys normally don't cause errors, so the example is a bit weird.

[alexislefebvre]

My bad, it's a warning: https://3v4l.org/SuJRd#vnull

It also trigger a deprecation, that is also silenced if the array has only one element (previous example).

I'll have to find something more problematic than using an invalid key because a warning is not a big deal. The example should mention an unrecoverable error.