log: describe how to see all revisions, point to revset docs

Perhaps it would be nice to point to the revset docs from every
argument that takes a revset, or perhaps that would be too
verbose. `jj log` is perhaps where most people first run into revset
syntax, so I hope pointing to the docs from there is a good start.

cli/src/commands/log.rs

@@ -43,7 +43,10 @@ use crate::ui::Ui;
 ///
 /// Renders a graphical view of the project's history, ordered with children
 /// before parents. By default, the output only includes mutable revisions,
-/// along with some additional revisions for context.
+/// along with some additional revisions for context. Use `jj log -r ::` to see
+/// all revisions. See `jj help -k revsets` (or
+/// https://martinvonz.github.io/jj/latest/revsets/) for information about the
+/// syntax.
 ///
 /// Spans of revisions that are not included in the graph per `--revisions` are
 /// rendered as a synthetic node labeled "(elided revisions)".

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

@@ -1336,7 +1336,7 @@ This excludes changes from other commits by temporarily rebasing `--from` onto `
 
 Show revision history
 
-Renders a graphical view of the project's history, ordered with children before parents. By default, the output only includes mutable revisions, along with some additional revisions for context.
+Renders a graphical view of the project's history, ordered with children before parents. By default, the output only includes mutable revisions, along with some additional revisions for context. Use `jj log -r ::` to see all revisions. See `jj help -k revsets` (or https://martinvonz.github.io/jj/latest/revsets/) for information about the syntax.
 
 Spans of revisions that are not included in the graph per `--revisions` are rendered as a synthetic node labeled "(elided revisions)".