[Repo Assist] docs: add missing (returns) XML tags to singleton, isEmpty, length, lengthOrMax, lengthBy, lengthByAsync#347

dsyme merged 3 commits intomainfsprojects/FSharp.Control.TaskSeq:mainfrom

repo-assist/improve-xmldoc-returns-tags-20260316-57d9cdc0fc84d55cfsprojects/FSharp.Control.TaskSeq:repo-assist/improve-xmldoc-returns-tags-20260316-57d9cdc0fc84d55cCopy head branch name to clipboard