Add an example of passing rustflags to docs

[Mark-Simulacrum]

This avoids confusion with the {} brackets being actually necessary, leading
to configuring a run in which they are directly passed to rustc.

[Mark-Simulacrum]

cc rust-lang/rust#85530 (comment) -- not yet handled case. Probably suggests switching away from {foo} to e.g. $foo might make sense across the whole docs.

[eddyb]

cc rust-lang/rust#85530 (comment) -- not yet handled case. Probably suggests switching away from {foo} to e.g. $foo might make sense across the whole docs.

FWIW what tripped me up in that case, was that crates=list:https://git.io/Jes7o was one of the examples (though with crates= implied, not explicit), but when using an URL to a text file, for that purpose, the list: part shouldn't actually be included (which makes sense, the : means it can be disambiguated from an URL).

[Mark-Simulacrum]

@bors r+

[bors]

📌 Commit b991f05 has been approved by Mark-Simulacrum

[bors]

⌛ Testing commit b991f05 with merge c899b6f...

[bors]

☀️ Test successful - checks-actions
Approved by: Mark-Simulacrum
Pushing c899b6f to master...