upgrading.dbk: small improvement to text about check for free disk

I have the feeling you're just writing what's already said in different words. I'm fine with revisiting the paragraph above after the bookworm release, but please elaborate what exactly you try to achieve.

i started from https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=992345 which i understood to say that you need to account for both the numbers when deciding if you can upgrade: i definitely did not understand this from the previous text (it might even be the difference between me being able to even attempt the next upgrade or having to reinstall/repartition - if i have misunderstood then i would be relieved of course!)

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I wonder if we should get an authoritative answer. We need to be sure here about what we are saying. I.e. which things does apt check for and warn against? One thing that caused me lots of failed upgrades is that sometimes an upgrade triggers two initramfs generations, which (in my case) happen in a small /boot partition. A kernel upgrade is going to happen during upgrade.