docs: document watcher debugger commands

doc/api/debugger.markdown

@@ -72,8 +72,58 @@ Then once the debugger is run, it will break on line 4.
 
 The `repl` command allows you to evaluate code remotely. The `next` command
 steps over to the next line. There are a few other commands available and more
-to come type `help` to see others.
+to come. Type `help` to see others.
 
+### Watchers
+
+You can watch expression and variable values while debugging your code.
+On every breakpoint each expression from the watchers list will be evaluated
+in the current context and displayed just before the breakpoint's source code
+listing.
+
+To start watching an expression, type `watch("my_expression")`. `watchers`
+prints the active watchers. To remove a watcher, type
+`unwatch("my_expression")`.
+
+### Commands reference
+
+#### Stepping
+
+* `cont`, `c` - Continue execution
+* `next`, `n` - Step next
+* `step`, `s` - Step in
+* `out`, `o` - Step out
+
+#### Breakpoints
+
+* `setBreakpoint()`, `sb()` - Set breakpoint on current line
+* `setBreakpoint('fn()')`, `sb(...)` - Set breakpoint on a first statement in
+functions body
+* `setBreakpoint('script.js', 1)`, `sb(...)` - Set breakpoint on first line of
+script.js
+* `clearBreakpoint`, `cb(...)` - Clear breakpoint
+
+#### Info
+
+* `backtrace`, `bt` - Print backtrace of current execution frame
+* `list(5)` - List scripts source code with 5 line context (5 lines before and
+after)
+* `watch(expr)` - Add expression to watch list
+* `unwatch(expr)` - Remove expression from watch list
+* `watchers` - List all watchers and their values (automatically listed on each
+breakpoint)
+* `repl` - Open debugger's repl for evaluation in debugging script's context
+
+#### Execution control
+
+* `run` - Run script (automatically runs on debugger's start)
+* `restart` - Restart script
+* `kill` - Kill script
+
+#### Various
+
+* `scripts` - List all loaded scripts
+* `version` - Display v8's version
 
 ### Advanced Usage