Fix Base.empty!(::GenericModel) and document its troublesome behavior

[odow]

I'll hold off merging this one until we get a few eyes on it. The behaviour of references before and after empty! was surprising to me, even though it's a direct consequence of what we're doing.

[odow]

In particular, was anyone aware of this feature

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.96%. Comparing base (60cb1ee) to head (3e04eeb).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4237   +/-   ##
=======================================
  Coverage   99.96%   99.96%           
=======================================
  Files          43       43           
  Lines        6315     6317    +2     
=======================================
+ Hits         6313     6315    +2     
  Misses          2        2           

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