refactor(issue,pr): trim help, restating comments and single-use helpers

Cut issue, pr, label, milestone and comment examples to at most three, drop
doc comments that only restate the name, fold the comment and attachment
argument lookups into one closure each, and inline label helpers with one
caller.

Signed-off-by: bircni <bircni@icloud.com>

internal/app/discussion_search.go

@@ -24,7 +24,6 @@ func (a *application) discussionSearchCommand(pulls bool) *cobra.Command {
 	var updated timeRange
 	opts.Pulls = pulls
 	example := fmt.Sprintf(`$ tea search %[1]s "login error"
-$ tea search %[1]s --author @me --state all
 # Search one repository; its labels and milestones are checked
 $ tea search %[1]s crash -R alice/project --label bug --milestone v1.0`, noun)
 	if pulls {