pivot_root.2: Minor wording tweaks

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

man2/pivot_root.2

@@ -51,9 +51,10 @@ and
 must not be on the same mount as the current root.
 .IP \-
 \fIput_old\fP must be at or underneath \fInew_root\fP;
-that is, adding a nonnegative
-number of \fI/..\fP to the string pointed to by \fIput_old\fP must yield
-the same directory as \fInew_root\fP.
+that is, adding some nonnegative
+number of "\fI/..\fP" prefixes to the pathname pointed to by
+.I put_old
+must yield the same directory as \fInew_root\fP.
 .IP \-
 .I new_root
 must be a path to a mount point, but can't be
@@ -165,12 +166,13 @@ or current working directory on the old root directory to the
 new root frees the old root directory of users,
 allowing the old root mount to be unmounted more easily.)
 .PP
-A typical use of
+One use of
 .BR pivot_root ()
 is during system startup, when the
-system mounts a temporary root filesystem (e.g., an \fBinitrd\fP), then
-mounts the real root filesystem, and eventually turns the latter into
-the current root of all relevant processes or threads.
+system mounts a temporary root filesystem (e.g., an
+.BR initrd (4)),
+then mounts the real root filesystem, and eventually turns the latter into
+the root directory of all relevant processes and threads.
 A modern use is to set up a root filesystem during
 the creation of a container.
 .PP
@@ -179,7 +181,7 @@ The fact that
 modifies process root and current working directories in the
 manner noted in DESCRIPTION
 is necessary in order to prevent kernel threads from keeping the old
-root directory busy with their root and current working directory,
+root mount busy with their root and current working directories,
 even if they never access
 the filesystem in any way.
 .PP