Add tooltip to timeline selector (#5842)

### What
* Closes #5836
* Blocked by emilk/egui#4338


![image](https://github.com/rerun-io/rerun/assets/1148717/0e53005a-7f75-4239-b20e-1cbddb57451a)


### Checklist
* [x] I have read and agree to [Contributor
Guide](https://github.com/rerun-io/rerun/blob/main/CONTRIBUTING.md) and
the [Code of
Conduct](https://github.com/rerun-io/rerun/blob/main/CODE_OF_CONDUCT.md)
* [x] I've included a screenshot or gif (if applicable)
* [x] I have tested the web demo (if applicable):
* Using newly built examples:
[rerun.io/viewer](https://rerun.io/viewer/pr/5842)
* Using examples from latest `main` build:
[rerun.io/viewer](https://rerun.io/viewer/pr/5842?manifest_url=https://app.rerun.io/version/main/examples_manifest.json)
* Using full set of examples from `nightly` build:
[rerun.io/viewer](https://rerun.io/viewer/pr/5842?manifest_url=https://app.rerun.io/version/nightly/examples_manifest.json)
* [x] The PR title and labels are set such as to maximize their
usefulness for the next release's CHANGELOG
* [x] If applicable, add a new check to the [release
checklist](https://github.com/rerun-io/rerun/blob/main/tests/python/release_checklist)!

- [PR Build Summary](https://build.rerun.io/pr/5842)
- [Recent benchmark results](https://build.rerun.io/graphs/crates.html)
- [Wasm size tracking](https://build.rerun.io/graphs/sizes.html)

crates/re_time_panel/src/time_control_ui.rs

@@ -2,7 +2,7 @@ use egui::NumExt as _;
 
 use re_entity_db::TimesPerTimeline;
 use re_log_types::TimeType;
-use re_ui::UiExt as _;
+use re_ui::{list_item, UiExt as _};
 
 use re_viewer_context::{Looping, PlayState, TimeControl};
 
@@ -39,7 +39,32 @@ impl TimeControlUi {
                             time_control.set_timeline(*timeline);
                         }
                     }
-                });
+                })
+                .response
+                .on_hover_ui(|ui| {
+                    list_item::list_item_scope(ui, "tooltip", |ui| {
+                        ui.markdown_ui(
+                            egui::Id::new("timeline_selector_tooltip"),
+                            r"
+Select timeline.
+
+Each piece of logged data is associated with one or more timelines.
+
+The logging SDK always creates two timelines for you:
+* `log_tick` - a sequence timeline with the sequence number of the log call
+* `log_time` - a temporal timeline with the time of the log call
+
+You can also define your own timelines, e.g. for sensor time or camera frame number.
+"
+                            .trim(),
+                        );
+
+                        ui.re_hyperlink(
+                            "Full documentation",
+                            "https://rerun.io/docs/concepts/timelines",
+                        );
+                    });
+                })
         });
     }