kbuild: update comment block of scripts/clang-version.sh

Commit 469cb737 ("kconfig: add CC_IS_CLANG and CLANG_VERSION")
changed the code, but missed to update the comment block.

The -p option was gone, and the output is 5-digit (or 6-digit when
Clang 10 is released).

Update the comment now.

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