Fix two bugs in filesystem port of DocumentCommandHandlers:
* 'Close Others' with one unsaved file did nothing (after choosing to
save) -- this was a merge error in doSave().
* 'Close Others' with multiple unsaved files doesn't close working set
items that have never been opened yet (after choosing to save) -- appears
to be a bug in the code we copied from PR #5497 (saveFileList() &
_doCloseDocumentList() were not on the same page about the list of files
meant that was returned from saveFileList()). Added a note in the PR.

Also rename 'savedFiles' vars to avoid confusion over whether they include
files that weren't dirty. And improve docs.