bug#23877: [Guix Info][7.1.4] Should remind user to do "guix pull"

[Eus]

Hi guys!

I downloaded Guix for USB disk from the Guix download page.

Then, when I did "guix system init /mnt/etc/config.scm /mnt" following the Guix 
info available at Alt+F2, I got an error suggesting me to use "--fallback" 
option.

I did that and my machine started to build sources. Then, I hit some error when 
building webkitgtk-2.12.0 and went to the Guix IRC channel. There two users 
told me that I should do "guix pull" first so that I do not have to build from 
source.

So, I think it is best to mention about "guix pull" in the Guix info page 
section 7.1.4.

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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