kbuild: update comments in scripts/Makefile.modpost

The comment line "When building external modules ..." explains
the same thing as "Include the module's Makefile ..." a few lines
below.

The comment "they may be used when building the .mod.c file" is no
longer true; .mod.c file is compiled in scripts/Makefile.modfinal
since commit 9b9a3f20 ("kbuild: split final module linking out
into Makefile.modfinal"). I still keep the code in case $(obj) or
$(src) is used in the external module Makefile.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>