List 'help' and '?' commands in documentation

book/src/cli-usage.md

@@ -37,6 +37,7 @@ There is a set of special commands that only work in interactive mode:
 |---------|--------|
 | `list`, `ll`, `ls` | List all constants, units, and dimensions |
 | `clear` | Clear screen |
+| `help`, `?` | View short help text |
 | `quit`, `exit` | Quit the session |
 
 ### Key bindings

book/src/web-usage.md

@@ -25,6 +25,7 @@ There is a set of special commands that only work in the web version:
 | Command | Action |
 |---------|--------|
 | `list`, `ll`, `ls` | List all constants, units, and dimensions |
+| `help`, `?` | View short help text |
 | `reset` | Reset state (clear constants, functions, units, …) |
 | `clear` | Clear screen |