Mention .PHONY in make clean section

[jxu]

.PHONY should be mentioned in the make clean section, as while it is mentioned much later on in the article, clean is where it's commonly seen. https://www.gnu.org/software/make/manual/html_node/Phony-Targets.html

make all section should also include it as well for completeness (should there be a file named all for some reason)

[Chase Lambert]

You're correct, but I don't want to add it to the tutorial because:

• In practice, many people don't use .PHONY because of how rare files called all or clean are
• It will clutter up every example with a detail that's minimally important

Nevertheless, I've now pointed this point out explicitly. See 06e5b9d

Thanks!