Naming, layout and comments: descriptive full-word names, one item per line in wrapped calls and literals, and doc comments on exported names

Short and single-letter names in application code become full words (tests
keep theirs). Wrapped calls, signatures and composite literals put one item
per line, and one-line function bodies are expanded. Exported names and
groups of constants get doc comments, and comments refer to the renamed
identifiers.

internal/fetcher/fetcher.go

@@ -12,6 +12,8 @@ import (
 	"git.avsdev.uk/Go/job-search/internal/util"
 )
 
+// ErrNoNewReleases is returned by a source's task when there was nothing new
+// to fetch; it wraps util.ErrWorkerNoWork.
 var ErrNoNewReleases = fmt.Errorf("%w: no new releases", util.ErrWorkerNoWork)
 
 // Option configures Run.