Enhance regen_doc.g

[ssiccha]

• adds a table of all unused methods to the manual
• at the start of each method, insert by which database it is used

Fixes #257.

[codecov[bot]]

Codecov Report

Merging #262 (0eb98fa) into master (6dcc0f8) will not change coverage. The diff coverage is n/a.

:exclamation: Current head 0eb98fa differs from pull request most recent head f9ea902. Consider uploading reports for the commit f9ea902 to get more accurate results

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##           master     #262   +/-   ##
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  Coverage   77.76%   77.76%           
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  Files          37       37           
  Lines       17497    17497           
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  Hits        13606    13606           
  Misses       3891     3891           

[ssiccha]

@FriedrichRober @danielrademacher What we talked about today during the call.

[ssiccha]

@fingolfin I've improved the "used for ... groups" strings. I'll squash everything in the end.