Style: improve ONLY_FOR_ARCHS_REASON (grammar, markup, etc.)

ONLY_FOR_ARCHS_REASON is used as part of the sentence and thus should
start with lower-case letter and not end with a period which is added
by the framework, similar to other knobs like BROKEN, IGNORE, et al.

While here, remove needless quoting, add missing Oxford comma, expand
contractions and jargonisms, use correct spelling for proper names.