[docs] [issue18384] Add devhelp build instructions to the documentation makefile

[R. David Murray]

R. David Murray added the comment:

I'm not sure that this belongs in core, but I'll certainly defer to Georg on that :).  If it does go in, the help string should explain what 'devhelp' does, since it isn't obvious from the name (for a non-GNOME user...I had to google it).

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nosy: +r.david.murray

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