Document BlockData.clone

[jsiirola]

Fixes # .

Summary/Motivation:

This adds missing documentation for BlockData.clone(), motivated by answering a SO question

Changes proposed in this PR:

• Document BlockData.clone()

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1. I agree my contributions are submitted under the BSD license.
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[mrmundt]

I think it's missing a comma for grammatical-correctness but this looks great.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.70%. Comparing base (fc9559f) to head (e9f42b4).
Report is 7 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3542   +/-   ##
=======================================
  Coverage   88.70%   88.70%           
=======================================
  Files         888      888           
  Lines      101963   101963           
=======================================
+ Hits        90446    90450    +4     
+ Misses      11517    11513    -4     

Flag	Coverage Δ
builders	26.55% <ø> (+<0.01%)	⬆️
default	84.81% <ø> (?)
expensive	33.94% <ø> (?)
linux	86.17% <ø> (-2.29%)	⬇️
linux_other	86.17% <ø> (-0.01%)	⬇️
osx	76.08% <ø> (ø)
win	84.66% <ø> (+<0.01%)	⬆️
win_other	84.66% <ø> (+<0.01%)	⬆️

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