Improve the documentation of jj prev

This will hopefully make it clear that `jj prev` does not
move by [OFFSET] relative to `@`, which is a misconception
that I had and I think others may also have.

I am suggesting this change as a result of the vigorous discussion in
these two issues:

- #3426
- #3445

We should make similar changes to `jj next` as well since
it follows similar rules.

cli/src/commands/prev.rs

@@ -21,10 +21,15 @@ use crate::command_error::{user_error, CommandError};
 use crate::commands::next::choose_commit;
 use crate::ui::Ui;
 
-/// Move the working-copy commit to the parent revision
+/// Move the working-copy commit relative to the parent revision
 ///
 ///
-/// The command moves you to the parent in a linear fashion.
+/// The command moves `@-` (and therefore `@`) to an ancestor in a linear
+/// fashion. If you run `jj prev n` and `--edit` is not used or implied, then
+/// `jj prev` will operate as if `@` had been moved to its parent and then moved
+/// backwards by `n` revisions before running `jj new` to create a new commit.
+/// If `@-` resolves to multiple revisions, then the command fails.
+///
 ///
 /// ```text
 /// D @  D

cli/tests/cli-reference@.md.snap

@@ -126,7 +126,7 @@ To get started, see the tutorial at https://github.com/martinvonz/jj/blob/main/d
 * `next` — Move the working-copy commit to the child revision
 * `obslog` — Show how a change has evolved
 * `operation` — Commands for working with the operation log
-* `prev` — Move the working-copy commit to the parent revision
+* `prev` — Move the working-copy commit relative to the parent revision
 * `rebase` — Move revisions to different parent(s)
 * `resolve` — Resolve a conflicted file with an external merge tool
 * `restore` — Restore paths from another revision
@@ -1347,10 +1347,15 @@ This restores the repo to the state at the specified operation, effectively undo
 
 ## `jj prev`
 
-Move the working-copy commit to the parent revision
+Move the working-copy commit relative to the parent revision
 
 
-The command moves you to the parent in a linear fashion.
+The command moves `@-` (and therefore `@`) to an ancestor in a linear
+fashion. If you run `jj prev n` and `--edit` is not used or implied, then
+`jj prev` will operate as if `@` had been moved to its parent and then moved
+backwards by `n` revisions before running `jj new` to create a new commit.
+If `@-` resolves to multiple revisions, then the command fails.
+
 
 ```text
 D @  D