obsolete docs in hook_form_BASE_FORM_ID_alter()

Problem/Motivation

This is D7 code!

 * By default, when \Drupal::formBuilder()->getForm() is called, Drupal looks
 * for a function with the same name as the form ID, and uses that function to
 * build the form. In contrast, base forms allow multiple form IDs to be mapped
 * to a single base (also called 'factory') form function.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet