Auto merge of #4923 - wking:console-markup, r=alexcrichton

doc: Replace 'shell' language labels (generally with 'console')

#GitHub [uses Linguist][1] for syntax highlighting.  Linguist's [`shell` language][2] is for the *language* (e.g. the contents of a `.sh` file).  The proper language for a shell session is [`ShellSession`][3], although in this commit I've used the [alias `console`][4].

The Cargo book [uses mdBook][4.5], mdBook [uses Highlight.js][5], and Highlight.js [also supports `console` as an alias for shell sessions][6].

A handful of places where we had been using `shell` were just command output, not shell sessions (e.g. they lacked a prompt and command).  In this commit, I've left those without a language label.

[1]: https://help.github.com/articles/creating-and-highlighting-code-blocks/#syntax-highlighting
[2]: https://github.com/github/linguist/blob/v5.3.3/lib/linguist/languages.yml#L4208
[3]: https://github.com/github/linguist/blob/v5.3.3/lib/linguist/languages.yml#L4249
[4]: https://github.com/github/linguist/blob/v5.3.3/lib/linguist/languages.yml#L4255
[4.5]: https://github.com/rust-lang/cargo/tree/f60ece98c10aa3716d037524da4ee989e212fe19/src/doc#requirements
[5]: https://rust-lang-nursery.github.io/mdBook/format/theme/syntax-highlighting.html#syntax-highlighting
[6]: https://github.com/isagalaev/highlight.js/blob/9.12.0/src/languages/shell.js#L10

diff --cc src/doc/src/guide/dependencies.md
Simple merge

diff --cc src/doc/src/reference/specifying-dependencies.md
Simple merge