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/20231005171747.541123-1-paulmck@kernel.org/

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Arnd Bergmann <arnd@kernel.org>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

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