either add it as a FAQ or as part of Portage User Guide, but add something about how our masking works and how you get around it
it's in the developer 'how to write an ebuild' guide. could probably just copy and paste.
This is added in portage-user.xml and portage-manual.xml by carl. I'll mark this as fixed. If you think the added information is not sufficient, please unfix this bug :)