docs: use correct terminology for 1024 bytes

docs: use correct terminology for 1024 bytes

Yes, I like kilobytes better than kibibytes (when I say kilobytes,
I generally mean 1024).  But since the term is ambiguous, it can't
hurt to say what we mean, by using both the correct name and
calling out the numeric equivalent.

* src/libvirt.c (virDomainGetMaxMemory, virDomainSetMaxMemory)
(virDomainSetMemory, virDomainSetMemoryFlags)
(virNodeGetFreeMemory): Tweak wording.
* docs/formatdomain.html.in: Likewise.
* docs/formatstorage.html.in: Likewise.