Add precision for create_dir function

[GuillaumeGomez]

Took me a while to find the other equivalent so it seems to be necessary.

r? @QuietMisdreavus

[GuillaumeGomez]

Thanks for the polishing @QuietMisdreavus! Updated.

[QuietMisdreavus]

Oh hey, looking again, there's a perfectly usable "Errors" section that already exists, right below the note you added! How about adding this to that list instead?

- A parent directory does not exist. To create a directory and all its parent directories at the same time, use [`create_dir_all`].

[GuillaumeGomez]

Ah sure. The problem is that it looks exactly like the create_dir_all docs. I wanted to make it stand out. I can add both if you prefer?

[QuietMisdreavus]

"Looking the same as the other docs" is the point. If we have the note somewhere, people will be able to find it. I don't want to have two different sections that describe when a function returns an error; the one you're adding is just redundant. We have a perfectly serviceable documentation standard when describing the situations where a function returns an error, please use it.

[GuillaumeGomez]

Added it to the error list.

[QuietMisdreavus]

The other note is still there, but looking at it again, i think it's probably fine to leave it. If it seems redundant to enough other people, we can take it out later. Thanks!

@bors r+ rollup

[bors]

📌 Commit 424fecd has been approved by QuietMisdreavus