docs: Adjust globals documentation

[klmcadams]

• Clarify when to use app.update_globals(globals()) vs. app = App(globals=globals()) (If you want to exclude enums in global vars, you should use app.update_globals(globals(), False)
• Update cheatsheet with globals parameter

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.90%. Comparing base (46a4f1a) to head (238c48c).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1100      +/-   ##
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+ Coverage   79.71%   79.90%   +0.18%     
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  Files          43       43              
  Lines        2854     2856       +2     
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+ Hits         2275     2282       +7     
+ Misses        579      574       -5