[Yum] makecache not in man page?

[bunny]

Trying to RTFM. makecache is listed as a yum command, but nowhere is it
explained. What is it about? Where is the documentation?

 man yum|grep -e 'is one of' -e makecache
       command is one of:
        * makecache

Is there a more complete source of documentation than the man page that I
ought to be looking at for answers like this? Robert Brown's howto and
article at <
http://www.phy.duke.edu/~rgb/General/yum_HOWTO/yum_HOWTO/yum_HOWTO.html> and
<http://www.phy.duke.edu/~rgb/General/yum_article/yum_article/> seem nicely
done (though a bit old) and I intend to read through them, but the TOC and
index do not immediately indicate an answer to makecache questions.

The larger question, if you choose to address it, is... I am about to do a
yum update on a very critical machine. I am a little nervous that the update
might break something important. Is there a way to make a 'checkpoint' or
'snapshot' so that I can tell yum to put things back exactly how they were
before I made my regretable mistake?

I realize, after googling this a bit, that yum is never supposed to break
anything, assuming one has wisely avoided installing rogue/broken RPMs or
(horror!) non-RPM stuff. Sadly, and not due to my choice, the machine in
question has not complied with this simple and fairly reasonable
restriction. Any philosophical reflections on the occaisional necessity of
installing something from source?

Thanks,
TDB
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