rustdoc: Syntax highlighting doesn't work for TOML and YAML

[camelid]

Here are some examples:

Docs for structopt

Rendered

DOM

<pre><code class="language-toml">[dependencies]
structopt = { version = &quot;0.3&quot;, features = [ &quot;paw&quot; ] }
paw = &quot;1.0&quot;
</code></pre>

Docs for clap

Rendered

DOM

<pre><code class="language-yaml">name: myapp
version: &quot;1.0&quot;
author: Kevin K. &lt;kbknapp@gmail.com&gt;
about: Does awesome things
args:
    - config:
        short: c
        long: config
        value_name: FILE
        help: Sets a custom config file
        takes_value: true
    - INPUT:
        help: Sets the input file to use
        required: true
        index: 1
    - verbose:
        short: v
        multiple: true
        help: Sets the level of verbosity
subcommands:
    - test:
        about: controls testing features
        version: &quot;1.3&quot;
        author: Someone E. &lt;someone_else@other.com&gt;
        args:
            - debug:
                short: d
                help: print debug information
</code></pre>

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@rustbot modify labels: T-rustdoc C-bug

[camelid]

Based on the DOM, it seems like it's being propagated as the correct language (not plaintext or something), but it's not actually being highlighted.

[ollie27]

Duplicate of #43165