Commit 469cb7376c ("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>