Manual pages: unshare.1: clarify description and example for --mount=<path>

Manual pages: unshare.1: clarify description and example for --mount=<path>

The existing text is not quite accurate, and I recently injected an
error into the EXAMPLES. This patch fixes both issues.

The text in DESCRIPTION incorrectly states that the propagation type of
the parent mount must be "private". This is not accurate.  Rather, the
propagation type must be something *other than "shared"* (i.e.,
"private", "slave", or "unbindable").

In the EXAMPLES section, I added text that implies that if the
propagation type of the parent mount is "shared", then the child mount
created by --mount=<path> might propagate to another namespace.
Rather, in this situation, an error would result. Clarify that.

Signed-off-by: Michael Kerrisk (man-pages) <mtk.manpages@gmail.com>