Improve clarity in the squash workflow intro

[gthb]

Trying to put my beginner's eyes to good use, by nailing down what made them 😵‍💫 spiral...

• “a new empty change on top of that one” — what does “that one” refer to? No change had been mentioned.
• “into our change” — what does “our change” refer to? Is it the “that one” change, or the “new empty change”?
• “move changes from @ into the change where [...]” — and @ identifies a change. So we move changes from a change into a change?
• “the change where we described what to do” — does “the change” here refer to the act of adding a description, or to the thing we added a description to?

Stepping through them one by one, I think:

• “that one” refers to the latest revision, the one shown by jj log -n 1. So give that a name in the first step, “call it C”.
• “our change” refers to C
• it's helpful to avoid the word “change” when not referencing instances of the jj concept called Change
• instead of “the change where we described what to do”, best to just reference the name C.

So here are concrete suggestions to make this less confusing, at least to me. (I think I'm a pretty representative N=1 sample of “people very used to git, and very new to jj”.)