docs: code comments name the headings README has now (closes #724)

The README rewrite renamed or merged several headings that comments still
cited. Each comment now names the current heading.

The e2e workflow's citations of "In CI" are dropped, because no section has what
they pointed at.

The comment on withLock() no longer states a Firefox private window exception,
because the manifest keeps the extension out of those windows.

Two comments also take corrections from reviews: the cases with no total in
DeleteAddress's warning, and the screens that say a swap's amount could not be
decoded.

Model: opus-5-5

tests/support/navigatorLocks.js

@@ -5,7 +5,7 @@
 // browser's own does for the one call the extension makes,
 // `navigator.locks.request(name, callback)`. Every module a test file loads
 // shares it, as the extension's background and its pages share the browser's
-// (README, "Stored state and its version").
+// (README, "Stored data").
 //
 // Run before each test file by `setupFiles` in package.json.