debugger: add help document for watch(index)

Author: cola119

lib/internal/debugger/inspect_repl.js

@@ -92,6 +92,7 @@ breakOnNone           Don't pause on exceptions (this is the default)
 
 watch(expr)           Start watching the given expression
 unwatch(expr)         Stop watching an expression
+unwatch(index)        Stop watching an expression at specific index from watch list
 watchers              Print all watched expressions and their current values
 
 exec(expr), p(expr), exec expr, p expr