doc: update the info on sort -b and -k

doc: update the info on sort -b and -k

* doc/coreutils.texi (sort invocation): Mention in the description
of -b, that the locale can also affect whether blanks are significant.
Update the list of ordering options for a --key that cause it
to not inherit any global ordering options.
Update the list of ordering options that implicitly skip whitespace.