Editorial: Replace flag set/unset with true/false

Rather than "setting" and "unsetting" flags, treat flag properties
as booleans (either true or false).

Replace:
 * "Set foo flag" with "Set foo flag to true"
 * "Unset foo flag" with "Set foo flag to false"
 * "If foo is set, ..." with "If foo is true, ..."
 * "If foo is unset, ..." with "If foo is false, ..."

Also clarify a few places that refer to "the foo flag"
to be more explicitly "bar's foo flag".

No normative behavior changes.