Improve confusing wording in `tag` container attribute's description

[JohnDowson]

Serializing a structure like this

#[derive(Serialize, Deserialize)]
#[serde(tag = "kind"]
enum Foo {
    Bar(Bar),
}

#[derive(Serialize, Deserialize)]
#[serde(tag = "kind")]
struct Bar {
    baz: String
}

results in duplicated kind fields, which is consistent with the struct being tagged externally, rather than the described behaviour of

Serialize the struct's name (or value of serde(rename)) as a field with the given key, in front of all the real fields of the struct. which suggests that the tag would be prepended to the struct's fields.

[dtolnay]

The original text is better, in my opinion. "External" in the context of a tag attribute suggests "externally tagged", which means a specific thing and is not the representation that #[serde(tag = "kind")] struct Bar {…} uses.