Document createFromTimestamp() for DateTime and DateTimeImmutable

[mathroc]

disclaimer: first time contributing here and edited from github web IDE. I didn't check if it's remotely close to be OK yet

[Girgias]

Preliminary markup review.

[lacatoire]

Suggested change

	<refentry xml:id="datetimeimmutble.createfromtimestamp" xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink">
	<refentry xml:id="datetimeimmutable.createfromtimestamp" xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink">

[lacatoire]

maybe it could be interesting to add an example with a float as a parameter ?

[lacatoire]

same https://github.com/php/doc-en/pull/5103/changes#r2816668016

[lacatoire]

That would be great to add a section errors and seealso. The implementation throws DateRangeError for NaN or out-of-range float timestamps. Worth documenting it.