Waste of computing resources in pull requests that only change the ddoc

[BBasile via Digitalmars-d]

I don't know If it's a known problem but auto tester resources 
are lost by PR that only change or add DDOC comments. For example 
whole dmd+runtime+phobos can be rebuild 3 times per day on 8 
machines while it's not necessary.

I understand that's a side-effect of the doc being directly in 
the code, but...

Wouldn't be possible to run a background tool, based on 
libdparse, that
- detects changed lines in the diff.
- run the libdparse-based tool to see if changed lines are only 
in the ddoc.

Depending on the results, either only CyberShadow's doc builder 
would run or also the big auto-tester.