cli: simplify help about default "log -r"

Since we've moved the default log revset to config/*.toml at 3dab92d, we don't
have to repeat the default value. It can be queried by "jj config list". I also
split the help paragraphs.

cli/src/commands/log.rs

@@ -46,9 +46,10 @@ use crate::ui::Ui;
 /// rendered as a synthetic node labeled "(elided revisions)".
 #[derive(clap::Args, Clone, Debug)]
 pub(crate) struct LogArgs {
-    /// Which revisions to show. If no paths nor revisions are specified, this
-    /// defaults to the `revsets.log` setting, or `@ |
-    /// ancestors(immutable_heads().., 2) | trunk()` if it is not set.
+    /// Which revisions to show
+    ///
+    /// If no paths nor revisions are specified, this defaults to the
+    /// `revsets.log` setting.
     #[arg(long, short)]
     revisions: Vec<RevisionArg>,
     /// Show revisions modifying the given paths

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

@@ -1221,7 +1221,9 @@ Spans of revisions that are not included in the graph per `--revisions` are rend
 
 ###### **Options:**
 
-* `-r`, `--revisions <REVISIONS>` — Which revisions to show. If no paths nor revisions are specified, this defaults to the `revsets.log` setting, or `@ | ancestors(immutable_heads().., 2) | trunk()` if it is not set
+* `-r`, `--revisions <REVISIONS>` — Which revisions to show
+
+   If no paths nor revisions are specified, this defaults to the `revsets.log` setting.
 * `--reversed` — Show revisions in the opposite order (older revisions first)
 * `-n`, `--limit <LIMIT>` — Limit number of revisions to show