docs: update the `[keys]` docs (#3114)

This is related to:
- #3090
- #1906

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## No longer relevant:

~I often just hold the backspace key to clear the query line in Atuin,
and the change in #3090 makes it no longer possible: it accepts the
current history entry instead.~ 🤯

~It may be a muscle memory thing, but it also feels wrong to me for the
left key to accept the current entry. I'd prefer for it to do nothing
when the cursor is at the start of the line (I've set
`exit_past_line_start = false` in my config).~

~This PR makes the new behavior configurable: it adds an
`accept_past_empty_line` setting (naming is *hard*, feel free to find a
better name) which lets you disable it. It also updates the `[keys]`
docs.~

BTW wouldn't setting the `accept_past_line_start` and
`accept_with_backspace` settings to `true` (maybe by default) have the
same effect as what #3090 introduced?

## Checks
- [x] I am happy for maintainers to push small adjustments to this PR,
to speed up the review cycle
- [x] I have checked that there are no existing pull requests for the
same thing

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