bug#23878: [Guix Info][7.1.4] Should remind user to mount bind /tmp

[Eus]

Hi guys!

In section 7.1.4 of the info displayed at Alt+F2 terminal, it is better to 
suggest the user to mount bind /tmp/ under /mnt/tmp2 so as to have more free 
space in case the user needs to build from source, for example, when using 
"--fallback" option in "guix system" command.

That is because when I did "guix system --falback init /mnt/etc/config.scm 
/mnt", the build failed after about 15 hours because there was no more space in 
/tmp.

BTW, I am capable of making patches. So, if you'd like me to make a patch, just 
tell me the source tree to make a diff against.

--
Best regards,
Eus (FSF member #4445)

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