Manual pages: unshare.1: improve intro paragraphs

Manual pages: unshare.1: improve intro paragraphs

The intro paragraphs of this page are rather hard for a newcomer to
grok. The name of the underlying system call (and consequently the
name of the command) are "strange", but let's help the reader by
naming more clearly what unshare(1) does: creating new namespaces. In
addition, clarify and expand the details on making a namespace
persistent using bind mounts.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>