rustdoc: warn on improperly interleaved HTML/MD

[notriddle]

Enhances the rustdoc::invalid_html_tags lint to warn in cases where pulldown-cmark will produce invalid HTML because the Markdown and HTML are improperly nested.

As an extreme example, this code does not do what it looks like it does, and it doesn't produce a warning under current nightly, either:

Figure 1: an X
<svg viewBox="0 0 10 10">
<path
    d="M0 10L10 0M10 10L0 0"
    stroke="black"
>
</svg>

This pull request makes it warn:

error: unopened HTML tag `svg`
  --> $DIR/invalid-html-tags.rs:273:5
   |
LL |   /// Figure 1: an X
   |  _____-
LL | | /// <svg viewBox="0 0 10 10">
   | |     ---- does not match this unclosed tag
LL | | /// <path
LL | | ///     d="M0 10L10 0M10 10L0 0"
LL | | ///     stroke="black"
LL | | /// >
   | |____- because of this Markdown paragraph, which is interrupted by a block quote
LL |   /// </svg>
   |       ^^^^^^

See also: https://talk.commonmark.org/t/thoughts-on-changing-supported-html-syntax-to-include-the-dangling-closing-angle-bracket/3827

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