Insufficient documentation of changes in loginpage_idp_list() method

While peer-reviewing MDL-58618, I realized that the recent changes in the structure of data returned by auth plugin's loginpage_idp_list() method are not documented clearly.

• As a part of MDL-58090 (in the commit 60237253a23), Damyon modified auth/classes/output/login.php so that the IdP's icon can be defined via iconurl (moodle_url or string) instead of the previous icon (pix_icon instance).
• This change has been only vaguely mentioned in lib/upgrade.txt as a part of MDL-58220 (commit f59d81f487d)
• Further work in MDL-58400 (627ea5b10ea) refactored things a bit but the change was not respected in inline documentation at other places.