minor #16920 [Form] Fix mistake about the reason of the limitation of the timestamp in 32bit systems (Deuchnord)

This PR was submitted for the 5.4 branch but it was merged into the 4.4 branch instead.

Discussion
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[Form] Fix mistake about the reason of the limitation of the timestamp in 32bit systems

In the docs of the forms `DateType` field, we can read the following warning:

> If `timestamp` is used, `DateType` is limited to dates between Fri, 13 Dec 1901 20:45:54 GMT and Tue, 19 Jan 2038 03:14:07 GMT on 32bit systems. This is due to a [limitation in PHP itself](https://www.php.net/manual/en/function.date.php#refsect1-function.date-changelog).

The limitation is actually not specific to PHP, but related to the [Year 2038 problem](https://en.wikipedia.org/wiki/Year_2038_problem) that affects all 32bits systems.
Just proposing a change to the docs to correct the information. :)

(I'm sorry if it reminds another similar bug from the end of the last century 👀)

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Commits
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50075b1 Fix mistake about the reason of the limitation of the timestamp in 32bit systems

Author: javiereguiluz

reference/forms/types/options/_date_limitation.rst.inc

@@ -1,7 +1,7 @@
 .. caution::
 
     If ``timestamp`` is used, ``DateType`` is limited to dates between
-    Fri, 13 Dec 1901 20:45:54 GMT and Tue, 19 Jan 2038 03:14:07 GMT on 32bit
-    systems. This is due to a `limitation in PHP itself`_.
+    Fri, 13 Dec 1901 20:45:54 UTC and Tue, 19 Jan 2038 03:14:07 UTC on 32bit
+    systems. This is due to an integer overflow bug in 32bit systems known as the `Year 2038 problem`_.
 
-.. _limitation in PHP itself: https://www.php.net/manual/en/function.date.php#refsect1-function.date-changelog
+.. _Year 2038 problem: https://en.wikipedia.org/wiki/Year_2038_problem