kbuild: doc: split if_changed explanation to a separate section

The if_changed macro is currently explained in the section
"Commands useful for building a boot image", but the use of
if_changed is not limited to the boot image.

It is often used together with custom rules. Let's split it as a
separate section, and insert it after the "Custom Rules" section.

I slightly reworded the explanation, re-numbered to fill the <deleted>
section, and also fixed the broken indentation of the Note: part.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>