kernel-doc: handle a void function without producing a warning

Currently a void function can produce a warning:
main.c:469: warning: contents before sections

This one is from arch/x86/kernel/cpu/sgx/main.c (which is not included
in any produced kernel documentation output).

Handle this by setting $in_doc_sect to 1 whenever any recognized
document section name is processed.

Fixes: f624adef3d0b ("kernel-doc: limit the "section header:" detection to a select few")
Signed-off-by: Randy Dunlap <[e-mail]>
Cc: Jani Nikula <[e-mail]>
Cc: Jonathan Corbet <[e-mail]>
Cc: [e-mail]
Signed-off-by: Jonathan Corbet <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

scripts/kernel-doc

···
     }
 
     if (/$doc_sect/i) { # case insensitive for supported section names
+	$in_doc_sect = 1;
 	$newsection = $1;
 	$newcontents = $2;