Update DocBlocks and inline docucmentation for includes/network.php
Update code layout and logic in includes/settings.php
Rename some settings labels
Rename settings callback params

* 'field_cb' is now 'callback'
* 'sanitize_cb' is now 'sanitize_callback'
Update labeling and label styling of plugin settings
Follow-up to a50ff65 in updating settings labels
Follow-up to c6a4e15 in updating settings labels
Remove all settings section's information text to let the settings speak for themselves
Remove the Additional Settings settings section

Moves the (custom) login message field to the Access Settings section.
Fix missing displaying of custom network login message