Add docs to ListItem and its appendChild about tight lists

[robinst]

Fixes #399.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.76%. Comparing base (e60be15) to head (c126ad3).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...rk/src/main/java/org/commonmark/node/ListItem.java	50.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #403      +/-   ##
============================================
- Coverage     94.80%   94.76%   -0.04%     
  Complexity      326      326              
============================================
  Files           146      146              
  Lines          4795     4799       +4     
  Branches        688      688              
============================================
+ Hits           4546     4548       +2     
- Misses          134      136       +2     
  Partials        115      115              

Files with missing lines	Coverage Δ
...rk/src/main/java/org/commonmark/node/ListItem.java	84.61% <50.00%> (-15.39%)	⬇️

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