doc: addresses nits in string_decoder, url, util

- Only `@@toStringTag` affects `util.isError()`, this is the reason why
it uses `Object.prototype.toString.call(argument)` under the hood.
- Shows an actual Euro symbol for reference.
- Uses line-drawing characters for the URL chart & fixes the chart
borders.

PR-URL: https://github.com/nodejs/node/pull/7026


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>