Improve error message from write-rustdoc-write-lines check

[theotherphil]

The following files are not formatted:
Error: - "../content/learn/book/storing-data/required-components.md"

Run write_rustdoc_hide_lines.sh to automatically fix these errors.
Error: Process completed with exit code 1.

The mention of write_rustdoc_hide_lines.sh is enough of a hint to go and find out what's happening here, but it would be a lot clearer if this error message included an explanation of what the check is actually for.

I'll have a go at coming up with a better message.