Documentation: /proc/loadavg: add 3 more field descriptions

Update contents of /proc/loadavg: add 3 more fields.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: linux-doc@vger.kernel.org
Link: https://lore.kernel.org/r/fe55b139-bd03-4762-199b-83be873cf7dd@infradead.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Author: rddunlap

Documentation/filesystems/proc.rst

@@ -687,7 +687,10 @@ files are there, and which are missing.
  kcore        Kernel core image (can be ELF or A.OUT(deprecated in 2.4))
  kmsg         Kernel messages
  ksyms        Kernel symbol table
- loadavg      Load average of last 1, 5 & 15 minutes
+ loadavg      Load average of last 1, 5 & 15 minutes;
+                number of processes currently runnable (running or on ready queue);
+                total number of processes in system;
+                last pid created.
  locks        Kernel locks
  meminfo      Memory info
  misc         Miscellaneous