docs: wanted, dbtest, recommendation and playlists comments say what the code does (#6277)

The same rewrite as d191a282, for four packages whose comments carried the
most history: tuning stories, quotes from the operator, single-install
measurements in library_scale.go, and 'used to' accounts of bugs. The
constraint each story protected stays as a present-tense reason (why tier 3
is reduced not removed, why the randomised arms take an order seed, why the
seed artist is eligible in Songs-like). Comments naming system_cron.go and
config.RecommendationConfig, neither of which holds what they claimed, now
name the scheduler and internal/recsettings.

Checked: non-comment tokens identical to HEAD; gofmt clean.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/wanted/manual.go

@@ -15,11 +15,10 @@ import (
 	"git.fabledsword.com/bvandeusen/minstrel/internal/tags"
 )
 
-// Matching a library album by hand (#5579): the Collection's Unmatched
-// section. A person names the MusicBrainz
-// album an album is, by a link or a search, and it goes on the list as held;
-// or leaves it off for good, as a local band's album MusicBrainz will never
-// have.
+// Matching a library album by hand: the Collection's Unmatched section. A
+// person names the MusicBrainz album an album is, by a link or a search, and
+// it goes on the list as held; or leaves it off for good, as a local band's
+// album MusicBrainz will never have.
 
 const matchManual = "manual"