Missing documentation on btoa() and atob() functions

Current documentation on the Javascript objects available in the duktape runtime does not contain reference to the btoa() and atob() functions.

Page to update:

https://www.zabbix.com/documentation/5.0/manual/config/items/preprocessing/javascript/javascript_objects

Some reference to these methods can be found in the javascript preprocessing documentation for version 4.4 at: 

https://www.zabbix.com/documentation/4.4/manual/appendix/preprocessing/javascript

and the 5.0 documentation at:

https://www.zabbix.com/documentation/5.0/manual/config/items/preprocessing/javascript

But since their availability is not limited to preprocessing I propose they are documented in the javascript_objects section which is referenced by both the webhook and perprocessing documentation.