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

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

GitHub uses Linguist for syntax highlighting [1].  Linguist's 'shell'
language is for the *language* [2] (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 (src/doc/README.md), 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
[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