Never seen or heard of one. I'd like a "best practice" one too.I know some document the scheme. I never have as my forms I don't think are that complicated.
Something I did pick up from Google was to add a view to my forms called developer notes. It's a basic 3 coloumn table, name, date and description.
In here I add:
Any new rules that I've created and not used before i.e. reference a webpage.
Links to an external data sources i.e. sql.
Also how my name format convention is created and when
Form Status
Anything else that I think will be of use.
In addition, since when it comes to changing a form, I can easily note down my changes directly in this view of the form and it gets published along with the new form. Therefore there is no documentation library that I need to go to and update subsequently.
In my SP Libraries I add a Change Request List so that any changes can be noted their too. For bigger documentation I use MS document template.