Mention `copytruncate` as the preferred strategy to rotating files in the `file` sink

[binarylogic]

We should reflect #2774 (comment) in the file sink documentation.

[jszwedko]

This is mentioned under https://vector.dev/docs/reference/configuration/sources/file/#file-rotation

[vaskozl]

That documentation refers to the the file source not sink, which is what the original comment was about.

[jszwedko]

Good catch @vaskozl . Let me reopen this.