Update 4.5.md

Hi, this is a small contribution to the Installation docs, based on this discussions in the community forums:

* https://moodle.org/mod/forum/discuss.php?d=466563
* https://moodle.org/mod/forum/discuss.php?d=466603

Thanks for all the work you do for the Digital Commons.

Author: enrroquez

general/releases/4.5.md

@@ -23,6 +23,7 @@ These are just the minimum supported versions. We recommend keeping all of your
 - PHP extension **sodium** is required. See [Environment - PHP extension sodium](https://docs.moodle.org/en/Environment_-_PHP_extension_sodium).
 - PHP setting **max_input_vars** must be >= 5000. For further details, see [Environment - max input vars](https://docs.moodle.org/en/Environment_-_max_input_vars).
 - PHP variants: Only 64-bit versions of PHP are supported. *Note: Changed since 4.1*.
+- PHP pncntl_signal() function enabled required to run admin/cli/cron.php. Find here [more details](https://moodle.org/mod/forum/discuss.php?d=466563#p1873780).
 
 ### Database requirements