doc: align Buffer.concat documentation with behavior

[gurgunday]

Fixes #60400

• Fixes docs
• Adds more coverage

Cc @ChALkeR is there anything else that's not aligned?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.60%. Comparing base (2bda7cb) to head (146c0b5).
⚠️ Report is 19 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #60405      +/-   ##
==========================================
+ Coverage   88.56%   88.60%   +0.03%     
==========================================
  Files         704      704              
  Lines      207774   207774              
  Branches    40025    40036      +11     
==========================================
+ Hits       184022   184089      +67     
+ Misses      15807    15742      -65     
+ Partials     7945     7943       -2     

see 47 files with indirect coverage changes

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