update nonlocal-returns.md - add a note about -source:future compiler option

[unkarjedy]

No description provided.

[bishabosha]

I tested this now and it seems strange that no deprecation warning is produced at all, only an error with future (or migration warning with future-migration).

[unkarjedy]

FTR The warning doesn't appear in Scala 2 by default as well, until Xlint:nonlocal-return compiler option is provided

https://discord.com/channels/632150470000902164/632628489719382036/979406519659532298

[som-snytt]

It was deprecated 4c813c1

      if (!ctx.scala2Mode)
        ctx.strictWarning("Non local returns are deprecated; use scala.util.control.NonLocalReturns instead", tree.sourcePos)

but the mechanism is now errorOrMigration(from = future). Maybe that should be gradualErrorOrMigration(warnFrom = 3.0, errorFrom = 3.2).

Maybe because

[error] -- Error: .../dotty/compiler/src/dotty/tools/dotc/printing/RefinedPrinter.scala:265:35
[error] 265 |            case info: PolyType => return toTextMethodAsFunction(info)
[error]     |                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[error]     |Non local returns are no longer supported; use scala.util.control.NonLocalReturns instead
[error] one error found
[error] (scala3-compiler-bootstrapped / Compile / compileIncremental) Compilation failed