argv_split: fix kernel-doc warnings

Use proper kernel-doc notation to prevent build warnings:

lib/argv_split.c:36: warning: Function parameter or member 'argv' not described in 'argv_free'
lib/argv_split.c:61: warning: No description found for return value of 'argv_split'

Link: https://lkml.kernel.org/r/20230912060838.3794-1-rdunlap@infradead.org


Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>