DOC: expand on meaning of segment keyword in tessellation

[martinfleis]

closes #718

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.3%. Comparing base (4037c70) to head (e336b2a).
⚠️ Report is 126 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #720     +/-   ##
=======================================
+ Coverage   97.4%   98.3%   +0.9%     
=======================================
  Files         26      26             
  Lines       4328    4195    -133     
=======================================
- Hits        4214    4123     -91     
+ Misses       114      72     -42     

Files with missing lines	Coverage Δ
momepy/elements.py	97.9% <ø> (+1.0%)	⬆️

... and 3 files with indirect coverage changes

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