UI Reference: document icon helper functions and screen reader text

Created by: agh1

Reflects changes included in

• https://github.com/civicrm/civicrm-core/pull/17279
• https://github.com/civicrm/civicrm-core/pull/17255
• https://github.com/civicrm/civicrm-core/pull/17280
• https://github.com/civicrm/civicrm-core/pull/17283
• https://github.com/civicrm/civicrm-core/pull/17303
• https://github.com/civicrm/civicrm-core/pull/17285

The last two have not been merged as of the moment.