Documentation/process/coding-style.rst: don't use "extern" with function prototypes

`extern' with function prototypes makes lines longer and creates more
characters on the screen.

Do not bug people with checkpatch.pl warnings for now as fallout can be
devastating.

Link: http://lkml.kernel.org/r/20181101134153.GA29267@avx2
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/process/coding-style.rst

@@ -443,6 +443,9 @@ In function prototypes, include parameter names with their data types.
 Although this is not required by the C language, it is preferred in Linux
 because it is a simple way to add valuable information for the reader.
 
+Do not use the `extern' keyword with function prototypes as this makes
+lines longer and isn't strictly necessary.
+
 
 7) Centralized exiting of functions
 -----------------------------------