subscriber: Add note about potential misuse of stdout to documentation

[Natr1x]

Motivation

As noted in #2492 logging to stdout instead of stderr by default is just plain wrong.

It goes against established conventions and also increases the risk of bugs caused by mixing program output with diagnostics.

But since changing a default is (rightly) considered a breaking change this cannot be fixed until version 0.4.

Solution

Simply add a note to the documentation to make it very clear to users that the defaults are probably not what the are expecting and to discourage applications depending on tracing-subscriber from inheriting this default.