classic-ui: document global utils

[MrTango]

Document all global utils and global variables in templates.

The global variables in templates should be documented in classic-ui.
The rest of global utils should be documented in backend: https://6.docs.plone.org/backend/global-utils.html

read and verify: https://docs.plone.org/4/en/old-reference-manuals/plone_3_theming/buildingblocks/skin/templates/global-template-variables.html#global-template-variables

[stevepiercy]

@MrTango has this been completed in https://6.docs.plone.org/backend/global-utils.html ?