Feature summary (what you would like to be able to do and where):

I suggest making a separate system message at top of each preference tab. Many may be blank by default but a wiki could for example mention relevant gadgets which are on another tab.

Clicking through the enwiki preferences tabs with uselang=qqx shows that only three have a general system message at top:
Gadgets: gadgets-prefstext
Banners: centralnotice-user-prefs-intro
Beta features: betafeatures-section-desc

I think all the other tabs should also get a message so wikis can add important information. Make the new names systematic like gadgets-prefstext. If the tab anchor is mw-prefsection-foo then foo-prefstext like this:

User profile: personal-prefstext
Appearance: rendering-prefstext
Editing: editing-prefstext
Recent changes: rc-prefstext
Watchlist: watchlist-prefstext
Search: searchoptions-prefstext
Notifications: echo-prefstext

enwiki might use the Appearance message to say:
"See also the Appearance section under Gadgets."

Editing and Watchlist also have their own enwiki gadget sections. More advanced suggestion: If there is a gadget section with the same heading as the current tab then automatically link it in the default message with a "See also" like above.

Use case(s) (list the steps that you performed to discover that problem, and describe the actual underlying problem which you want to solve. Do not describe only a solution):

I'm an enwiki admin and wondered whether I could add messages referring to gadgets since many users will be unaware of those and not expect to find relevant options there. preferences-summary displays above all tabs but it cannot be customized by tab and many users wouldn't look there anyway for tab-specific content.

Benefits (why should this be implemented?):
Allow wikis to make helpful messages on preference tabs, e.g. links to relevant gadgets or help. Notifications at enwiki could link to https://en.wikipedia.org/wiki/Help:Notifications. The default might have a "Help" link to https://www.mediawiki.org/wiki/Special:MyLanguage/Help:Notifications. The "Muted" options already link sections on that page.