chore: Improve Initcap test and docs

[andygrove]

Which issue does this PR close?

Closes #1052

Rationale for this change

Clarify what is still incompatible.

What changes are included in this PR?

How are these changes tested?

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 57.43%. Comparing base (f09f8af) to head (ed92f6a).
⚠️ Report is 496 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #2387      +/-   ##
============================================
+ Coverage     56.12%   57.43%   +1.30%     
- Complexity      976     1297     +321     
============================================
  Files           119      147      +28     
  Lines         11743    13415    +1672     
  Branches       2251     2347      +96     
============================================
+ Hits           6591     7705    +1114     
- Misses         4012     4449     +437     
- Partials       1140     1261     +121     

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[kazuyukitanimura]

Thank you @andygrove