man: reword some sentences with umbiguous subjects

man: reword some sentences with umbiguous subjects

A sencence like "The system manager does, a, b, c, which is really d, and e.",
it is generally understood that the manager also does "e". This can be
quite confusing if the manager cannot do "e", in our case unmount the file
system on which it is sitting.

Similary, we cannot "fall back to x if it is missing", since "it" in that
sentence means "x".