> g.extension extension=i.fusion.hpf
> Downloading precompiled GRASS Addons <i.fusion.hpf>...
> Fetching <i.fusion.hpf> from <http://wingrass.fsv.cvut.cz/grass79/x86_64/addons/grass-7.9.dev/i.fusion.hpf.zip> (be patient)...
> Updating extensions metadata file...
> Updating extension modules metadata file...
> WARNING: No metadata available for module 'constants'.
> WARNING: No metadata available for module 'high_pass_filter'.
> ERROR: Unable to read manual page: [Errno 2] No such file or directory: 'C:\\Users\\youruser\\AppData\\Roaming\\GRASS7\\addons\\docs\\html\\constants.html'Well, does the file exist?
the file constants.html doesn't exist, but it's intentionally.
the related py-script lives in etc (C:\Users\youruser\AppData\Roaming\GRASS7\addons\etc\i.fusion.hpf\constants.py) as it is a python helper lib for i.fusion.hpf and therefore it has no manual html.
see
https://github.com/OSGeo/grass-addons/tree/master/grass7/imagery/i.fusion.hpf
and
https://github.com/OSGeo/grass-addons/blob/master/grass7/imagery/i.fusion.hpf/constants.py
ciao
helli