Documentation: virt: Clean up paravirt_ops doc

Clarify language. Clean up grammar. Hyphenate some words.

Change "low-ops" to "low-level" since "low-ops" isn't defined or even
mentioned anywhere else in the kernel source tree.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Acked-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20230610054310.6242-1-rdunlap@infradead.org

Documentation: move Documentation/virtual to Documentation/virt

Renaming docs seems to be en vogue at the moment, so fix on of the
grossly misnamed directories. We usually never use "virtual" as
a shortcut for virtualization in the kernel, but always virt,
as seen in the virt/ top-level directory. Fix up the documentation
to match that.

Fixes: ed16648eb5b8 ("Move kvm, uml, and lguest subdirectories under a common "virtual" directory, I.E:")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>