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: #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>

Author: Fishrock123

doc/api/string_decoder.md

@@ -28,8 +28,8 @@ internal buffer is used to ensure that the decoded string does not contain
 any incomplete multibyte characters. These are held in the buffer until the
 next call to `stringDecoder.write()` or until `stringDecoder.end()` is called.
 
-In the following example, the three UTF-8 encoded bytes of the European euro
-symbol are written over three separate operations:
+In the following example, the three UTF-8 encoded bytes of the European Euro
+symbol (`€`) are written over three separate operations:
 
 ```js
 const StringDecoder = require('string_decoder').StringDecoder;

doc/api/url.md

@@ -20,17 +20,17 @@ The following details each of the components of a parsed URL. The example
 illustrate each.
 
 ```
-+---------------------------------------------------------------------------+
-|                                  href                                     |
-+----------++-----------+-----------------+-------------------------+-------+
-| protocol ||    auth   |       host      |          path           | hash  |
-|          ||           +----------+------+----------+--------------+       |
-|          ||           | hostname | port | pathname |    search    |       |
-|          ||           |          |      |          +-+------------+       |
-|          ||           |          |      |          | |   query    |       |
-" http:    // user:pass @ host.com : 8080   /p/a/t/h  ? query=string  #hash "
-|          ||           |          |      |          | |            |       |
-+----------++-----------+-----------+------+----------+-+-----------+-------+
+┌─────────────────────────────────────────────────────────────────────────────┐
+│                                    href                                     │
+├──────────┬┬───────────┬─────────────────┬───────────────────────────┬───────┤
+│ protocol ││   auth    │      host       │           path            │ hash  │
+│          ││           ├──────────┬──────┼──────────┬────────────────┤       │
+│          ││           │ hostname │ port │ pathname │     search     │       │
+│          ││           │          │      │          ├─┬──────────────┤       │
+│          ││           │          │      │          │ │    query     │       │
+"  http:   // user:pass @ host.com : 8080   /p/a/t/h  ?  query=string   #hash "
+│          ││           │          │      │          │ │              │       │
+└──────────┴┴───────────┴──────────┴──────┴──────────┴─┴──────────────┴───────┘
 (all spaces in the "" line should be ignored -- they're purely for formatting)
 ```
 

doc/api/util.md

@@ -389,7 +389,7 @@ util.isError({ name: 'Error', message: 'an error occurred' });
 
 Note that this method relies on `Object.prototype.toString()` behavior. It is
 possible to obtain an incorrect result when the `object` argument manipulates
-the `@@toStringTag` or overrides the `toString()` method.
+`@@toStringTag`.
 
 ```js
 const util = require('util');