Add hints about HDD geometry issues, and spelling n*zi (spelling, typos, improvements)

Hi,

since I had lots of "fun" with partclone.ntfsfixboot on an Extended CHS partition table vs. a narrow LBA-only (*and thus *incompatible*) view of newer Linux kernels, I decided to add details and warnings about this to usage message / docs, to try to prevent others (the documentation-reading kind of users ;) from hitting the same annoying geometry-challenged NTFS-side non-boot issues that I did.

Note that the usage message changes are potential ntfsfixboot project upstream material
(perhaps it's better to do larger backports from time to time though).

Also, correct many typos/spelling/others (this work might partially conflict with a recent pull request).
About the spelling n*zi thing: yes, I'm German, that ought to be authoritative enough ;-P

Please let me know in case something cannot be accepted as is (it's my first pull request anyway). Partial application certainly is ok (even complete redoing of questionable parts). All that matters is that it gets into the project, pronto :)

Thanks a ton for a very useful tool!