doc: Update /proc/cmdline documentation to include boot config

Update the /proc/cmdline documentation to explicitly state that this
file provides kernel boot parameters obtained via boot config from the
kernel image as well as those supplied by the boot loader.

Link: https://lore.kernel.org/all/[email protected]/

Signed-off-by: Paul E. McKenney <[email protected]>
Acked-by: Masami Hiramatsu (Google) <[email protected]>
Reviewed-by: Randy Dunlap <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Stephen Rothwell <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Signed-off-by: Masami Hiramatsu (Google) <[email protected]>

Author: paulmckrcu

Documentation/filesystems/proc.rst

@@ -691,7 +691,8 @@ files are there, and which are missing.
  apm          Advanced power management info
  buddyinfo    Kernel memory allocator information (see text)	(2.5)
  bus          Directory containing bus specific information
- cmdline      Kernel command line
+ cmdline      Kernel command line, both from bootloader and embedded
+              in the kernel image
  cpuinfo      Info about the CPU
  devices      Available devices (block and character)
  dma          Used DMS channels