docs: Document I idle state in ps and top

Linux 4.2 provided a new process state of I which is used for an idle
kernel thread. This new state means that kernel threads do not
contribute to the loadavg as they are no longer state D or S but I.

While both ps and top displayed this state, it wasn't documented in
either manual page until now.

References:
 https://bugs.debian.org/886967
 https://www.quora.com/What-does-mean-Linux-process-state-I-in-the-top-output
 https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=80ed87c8a9ca0cad7ca66cf3bbdfb17559a66dcf

NEWS

@@ -1,5 +1,6 @@
 procps-ng-NEXT
 ----------------
+  * doc: Document I idle state in ps.1 and top.1           Debian #886967
   * kill: -l space between name parses correctly           Debian #854407
   * library: dont use vm_min_free on non Linux             Debian #831396
   * library: dont use SIGPWR on FreeBSD                    Debian #832148

ps/ps.1

@@ -4,7 +4,7 @@
 .\" Quick hack conversion by Albert Cahalan, 1998.
 .\" Licensed under version 2 of the Gnu General Public License.
 .\"
-.TH PS 1 "May 2017" "procps-ng" "User Commands"
+.TH PS 1 2018-01-13 "procps-ng" "User Commands"
 .\"
 .\" To render this page:
 .\"    groff -t -b -man -X -P-resolution -P100 -Tps ps.1 &
@@ -832,6 +832,9 @@ of a process:
 D
 uninterruptible sleep (usually IO)
 .TP
+I
+Idle kernel thread
+.TP
 R
 running or runnable (on run queue)
 .TP

top/top.1

@@ -837,6 +837,7 @@ The\fI real\fR user name.
 29.\fB S \*(Em Process Status \fR
 The status of the task which can be one of:
     \fBD\fR = uninterruptible sleep
+    \fBI\fR = idle
     \fBR\fR = running
     \fBS\fR = sleeping
     \fBT\fR = stopped by job control signal