Handbook update for 'make world'

[Mark Linimon]

I don't have any objection to any of this, but I think it might be
nice to at least put in the text a) that a change in recommendation
has been made, and b) why.  Something like 'the sequence that includes
"make buildworld/make installworld" is now preferred to the previously
documented usage "make world".  The latter skips some steps that may or
may not be necessary in certain circumstances; the former includes all
steps that are necessary in any circumstance, and is thus less likely
to lead to unsatisfactory results.'

mcl