Grease clarifications on sourcing_faq.md

[DavidCC00]

1. It's kind of unclear which is optimal/first choice.

2. It seems there's two versions of Superlube 21030. Some of them say "With PTFE" and some packages without. I just tried some "With PTFE" and got horrible results (was gritty and more friction than packed grease) and a few messages on the discord seem to indicate that particulate like PTFE is bad for linear rails. If this is the case, it should be added to the document.

Here's a picture on discord. https://cdn.discordapp.com/attachments/472450547534921729/1186792797790032012/image.png?ex=659489ec&is=658214ec&hm=a2b8a1ffbc1f83056d984d211a849f1df60f4fc1653e17a65e9bbf156733a14b&