doc,test: add information on unwatch(index) command

[cola119]

unwatch command of node inspect supports to remove specific index element.

node/lib/internal/debugger/inspect_repl.js

Lines 1030 to 1038 in 8d0f49c

	unwatch(expr) {
	const index = ArrayPrototypeIndexOf(watchedExpressions, expr);
	// Unwatch by expression
	// or
	// Unwatch by watcher number
	ArrayPrototypeSplice(watchedExpressions,
	index !== -1 ? index : +expr, 1);
	},

[cola119]

@Trott

About #43018 (review), I hope we can discuss here.
unwatch(index), not an expression, is implemented internally but undocumented anywhere. Should I add the doc to node help along with the debugger doc or leave it undocumented in terms of internal use only?

[Trott]

If it is clear how to obtain the index and how index differs from expr, and if this would be useful to end users, then we should document it. Thanks.

[cola119]

Removing watcher by index would be useful for end users because watchers command prints all watched expressions with their indices.

[cola119]

However, the current watch(expr) accepts numeric as well as string expression, which makes the difference index and expr ambiguous.
So I think string validation for watch(expr) is required (#42913).

[cola119]

@lpinca @Mesteery @Trott Could you check again and land if there is nothing wrong with.

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/44728/