[FIX] 1 line headers: only allow single-letter types and flags

[markotoplak]

Issue

Fixes #6945

Description of changes

This fix creates specific 1-line header matching rules (does not reuse the 3-line header ones).

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.48%. Comparing base (f70085b) to head (a32263a).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6959   +/-   ##
=======================================
  Coverage   88.47%   88.48%           
=======================================
  Files         332      332           
  Lines       73280    73287    +7     
=======================================
+ Hits        64834    64847   +13     
+ Misses       8446     8440    -6     

[janezd]

I named the groups. Otherwise OK; merge if you agree with my changes.

[markotoplak]

Thanks, naming groups is nice!