pivot_root.2: There is no restriction against 'put_old' being a mount point

pivot_root.2: There is no restriction against 'put_old' being a mount point

As far as I can see from the source code, the statement that
"No other filesystem may be mounted on 'put_old'" is incorrect.
Even looking at the 2.4.0 source code, there I can't see such
a restriction. In addition, some testing on a 5.0 kernel
(mounting 'put_old' in the new mount namespace just before
pivot_root()) did not result in an error for this case when
calling pivot_root().

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