man/systemctl: rework descriptions of bind and mount-image

man/systemctl: rework descriptions of bind and mount-image

The text used "unit's view" to mean mount namespace. But we talk about
mount namespaces in the later part of the paragraph anyway, so trying to
use an "approachable term" only makes the whole thing harder to understand.
Let's use the precise term.

Some paragraph-breaking and re-indentation is done too.

(cherry picked from commit e04eae5e1c43c050e0707d3fcfdc16691b761d61)