Reword the Expressions chapter of the docs

[encukou]

Continuing after #127833 and #135676, I'd like to reword and restructure the Expressions section of the Reference documentation.

I think I found a style that works: each section will start with the most obvious things, expand toward a complete & precise description, with examples along the way, and end with the formal grammar. (Currently the grammar is usually listed first, and the author of the prose is tempted to assume that the reader has parsed and understood it.)

Linked PRs

• gh-141984: Reword and reorganize Subscription (and Slicing) docs #141985