[ENH] Add filename template legend

[Remi-Gau]

closes #1198
replaces #1219

let's see how this one goes

[codecov]

Codecov Report

Merging #1259 (649e4e4) into master (b3afa2a) will decrease coverage by 0.09%.
The diff coverage is 77.77%.

@@            Coverage Diff             @@
##           master    #1259      +/-   ##
==========================================
- Coverage   88.33%   88.24%   -0.10%     
==========================================
  Files           6        6              
  Lines        1020     1029       +9     
==========================================
+ Hits          901      908       +7     
- Misses        119      121       +2     

Impacted Files	Coverage Δ
tools/schemacode/bidsschematools/render.py	92.83% <77.77%> (-0.35%)	⬇️

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[effigies]

Thank you for paring it down! It is much easier to think about when it's just the heart of it. Some small suggestions that should have no impact on the render but tighten up the function a bit.

[tsalo]

💯 😄

[sappelhoff]

Thanks @Remi-Gau