ebuild-writing/variables: add QA control variables section

[ingenarel]

these variables and their docs can be found if you do man 5 ebuild,

however they're not in the devmanual itself

this pr should fix that

originally found it out because i was working on this: mfussenegger/nvim-lint#904

[ulm]

Please read:
https://devmanual.gentoo.org/appendices/devbook-guide/index.html#internal-coding-style
and:
https://www.gentoo.org/glep/glep-0076.html#certificate-of-origin

[ingenarel]

ok i recommitted with the proper --signoff, fixed word wrapping to 80 chars as per the coding style, and added the necessary changes, if anything else is needed please lemme know

[ingenarel]

okay i have also properly did the note section

and also did <c></c> for the few stuff after some consideration:

• "automake missing --run command" line, the --run
• .GCC.command.line
• .hash
• SONAME

please lemme know if anything else is needed

[sbraz]

Can we please reword to avoid contain/containing in the same sentence?

[ingenarel]

done

[thesamesam]

Looks okay to me. I'd like to avoid these "This should contain..." but it's not something I insist on you fixing.

[ulm]

Just make all of these "A list of ...". No need for "This should be" or "This should contain", it doesn't transport any additional information.

[ulm]

Thanks, merged.