bug#26170: documentation: Explanation of propagated-inputs unclear

["pelzflorian (Florian Pelz)" <pelzflorian@pelzflorian.de>]

Hello,

I presume the difference between a package’s `inputs` and
`propagated-inputs` in Guix packages is that propagated inputs get
installed to a Guix profile whenever the package is installed while
inputs are dependencies that remain in the Store. I presume libraries
the packaged program is linked to do not need to go to
`propagated-inputs` because the program is linked to the files in the Store.

For more context, I am attempting to package the GNOME Evolution mail
client which looks for evolution-data-server’s
share/dbus-1/services/org.gnome.evolution.dataserver.Sources.service
at run time and thus should use it as a propagated input (I believe).

I am however confused by `info guix` where I find explanations like
“Lastly, ‘propagated-inputs’ is similar to ‘inputs’, but the
          specified packages will be automatically installed alongside
          the package they belong to (*note ‘guix package’:
          package-cmd-propagated-inputs, for information on how ‘guix
          package’ deals with propagated inputs.)”
and
“Sometimes packages have “propagated inputs”: these are dependencies
     that automatically get installed along with the required package
     (*note ‘propagated-inputs’ in ‘package’ objects:
     package-propagated-inputs, for information about propagated inputs
     in package definitions).“

I suggest mentioning more explicitly
 1) that `propagated-inputs` are automatically installed *to the Guix
profile* and not just the Store like regular inputs and
 2) that C/C++ libraries do not need to be propagated because they can
just as well be loaded from the Store *unless* their header files are
included by header files of another input package (?) and
 3) more examples like the above example for GNOME Evolution (which
however I have yet to finish packaging and submit as a patch; maybe
dconf is a better example – I presume it is also needed at run time and
not just).

This would have made the purpose of `propagated-inputs` easier to
understand.

Regards,
Florian