Clarify PTFE tube length

I was confused by the manual which did not specify the required length of the PTFE tube. Only after searching issues here did I understand from comments that different hotend mounts can lead to different lengths, so there is no single length that could be specified. But as there have been several issues on this topic, the text in the manual seems to be causing confusion. Could we clarify it?

For example, a simple statement like this would help. "The PTFE tube should stick up out of the hotend mount by 30mm."

The current text reads:

> There are 2 methods you can use to setup the PTFE tube. If you are swapping from CW2
then you will already have a PTFE tube you can just add to that with a new 19mm piece or
you can use the PTFE cutting jig in the G2 repo and cut a new piece. The new piece should
have 30mm sticking out the top of the hotend mount.

This text appears to suggest that just cutting a new 19mm piece and placing it on top would be sufficient. That seems like a bad idea because the two parts would be loose and potentially misalign. If that's the intention, let's say that. It's also confusing because it's not the new piece that should have 30mm; it's the new and old pieces together that should have 30mm.

A rewrite could be:

> After full insertion into your hotend mount, the PTFE tube should rise 30mm above it.
>
> If you already have a PTFE tube in your hotend mount, you can just add a new piece so that the total height is 30mm. That piece can be inserted loosely in the G2 body and will align sufficiently.
>
> You can use the PTFE cutting jig in the G2 repo and ensure the total rise of the PTFE tube is 30mm.

Start with the manual passage quoted in the issue and locate its source in the repository. Review the wording against the described hotend-mount variations and the proposed clarification, then verify that the updated instructions clearly define the total 30mm rise and the handling of existing and new tube pieces.

Written by the indexing model from the issue text.