blkif interface comment cleanups.  No functional changes
blkif interface comment cleanups.  No functional changes

sys/xen/interface/io/blkif.h:
 o Insert space in "Red Hat".
 o Fix typo "discard-aligment" -> "discard-alignment"
 o Fix typo "unamp" -> "unmap"
 o Fix typo "formated" -> "formatted"
 o Clarify the text for "params".
 o Clarify the text for "sector-size".
 o Clarify the text for "max-requests" in the backend section.

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