kbuild: doc: remove outdated description of the limitation on -I usage

Kbuild used to manipulate header search paths, enforcing the odd
limitation of "no space after -I".

Commit cdd750bf ("kbuild: remove 'addtree' and 'flags' magic for
header search paths") stopped doing that. This limitation no longer
exists. Instead, you need to accurately specify the header search path.
(In this case, $(src)/include)

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <n.schier@avm.de>