Seems to me that the SQL back end is likely to stay a feature of GnuCash. ;) Given that it is part of the application, I propose that documentation be drawn up to explain this aspect of the program. Said documentation will need to explain the features, the limitations, as well as the ongoing developmental nature of the SQL back ends. Presumably, such documentation will include elements in both the Help file and the Tutorial. The Tutorial has a section (2.5 Storing Your Financial Data) that is a logical location; I am not so sure about the Help file, and welcome suggestions. Text for this can come from the online FAQ as well as the list archives.
Right, from reading http://code.gnucash.org/docs/C/gnucash-guide/basics-files1.html I think, it is missing a bunch of things: First a warning, using a real DBMS requires knowledge, how to maintain a DBMS, at least, how to create backups. From a guide I would expect a suggestion, which one to use - special as newby. I think, the tutorial should concentrate on using a local XML file. The following http://code.gnucash.org/docs/C/gnucash-guide/basics-backup1.html and http://code.gnucash.org/docs/C/gnucash-guide/basics-migrate-settings.html only explain xml files. The right place might be a new chapter in help before "6. Common Transaction Operations" and linked by http://code.gnucash.org/docs/C/gnucash-help/ch04s02.html#AccTree-FileMenu and the save-as-dialog of the program. It should discuss 1. File formats and their requirements 2. File locations a. on the local desktop, (sync tools prefer uncompressed files) b. on a server .1 in the LAN (lock problems dependend on used protocoll) .2 in the cloud (use encryption) For your inspiration you might see http://wiki.gnucash.org/wiki/FAQ#Basics:_Filenames.2C_Directories.2C_... and google translate http://wiki.gnucash.org/wiki/De/FAQ#GnuCash_Dateien_und_Pfade as I am not shure everything was ported.
Oops, it should be in a Chapter "File Menu" and before Chapter 5. Setting Up, Editing & Working with Accounts
Guide update in PR #109.
According https://github.com/Gnucash/gnucash-docs/pull/109#issuecomment-419128963 it was merged on 6 Sep 2018. Thanks, David for your patience with us!