[authors] PING reviewers: Ch. 6 of Migr. Guide (Calc/Excel differences) ready for review

[RLShadow]

I have made a few more, mostly minor, edits to this document and have uploaded it to the Feedback page:   http://oooauthors.org/en/authors/userguide2/migration/feedback/   I would appreciate it if a few of you knowledgeable about Calc and Excel (and some that aren't!) could review it; thanks.   Dick Detwiler