cleanup and clarify documentation for write-only workbooks
Created originally on Bitbucket by Daniel Lenski
Was already merged in Bitbucket before import, marked as merged by the import user
Write-only documentation had an incomplete example, referred to the obsolete
optimized_write parameter, and left out a few important features of write-only workbooks.
(Replacing pull request #121 (closed) with a PR based on the 2.3 branch.)