fs: Warn about performance pitfall

[NotAFile]

Motivation

Straightforwardly porting synchronous filesystem code to tokio::fs can cause unexpected performance issues (and with tracing, memory issues too), if one is not aware of the implications of adapters spawning a new task for every operation.

Solution

Document the pitfall and provide a replacement example for the common action of reading a file line by line.

[Darksonn]

Thanks!