Update sample_rand docs for TwP (#12484)

* Update `sample_rand` docs for TwP

* Apply suggestions from code review

Co-authored-by: Alex Krawiec <[email protected]>

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Co-authored-by: Alex Krawiec <[email protected]>

Author: adinauer

Diff for: develop-docs/sdk/telemetry/traces/index.mdx

@@ -231,9 +231,10 @@ To improve the likelihood of capturing complete traces when backend services use
 The random value (`sample_rand`) is set according to the following rules:
 
 1. When tracing is enabled and a new trace is started, the SDK generates a `sample_rand` value for the current execution context. A `sample_rand` is a float (`0.1234` notation) in the range of `[0, 1)` (including 0.0, excluding 1.0).
-1. It is _recommended_ to generate the random number deterministically using the trace ID as seed or source of randomness. The exact method by which the random number is created is implementation defined and may vary between SDK implementations.
-1. The `sample_rand` is part of the DSC (Dynamic Sampling Context) and as with other values on the `baggage` header, the `sample_rand` value from the current execution context should be propagated to downstream SDKs.
-1. On incoming traces, an SDK takes the incoming `sentry-sample_rand` value in the `baggage` header and uses it for the rest of the current execution context (e.g. request).
+  1. This also applies to Tracing without Performance where we simply want to generate `sample_rand` every time a new trace is started. This `sample_rand` value can then be stored on `PropagationContext`.
+1. It is _recommended_ to generate the random number deterministically using the trace ID as seed or source of randomness (make sure to do this atomically so multiple threads accessing the same random instance don't mix their seeds). The exact method by which the random number is created is implementation-defined and may vary between SDK implementations.
+1. The `sample_rand` is part of the DSC (Dynamic Sampling Context) and as with other values on the `baggage` header, the `sample_rand` value from the current execution context should be propagated to downstream SDKs. It should also be sent to other system as part of the `baggage` header if Performance is disabled and sampling decision is deferred.
+1. On incoming traces, an SDK takes the incoming `sentry-sample_rand` value in the `baggage` header and uses it for the rest of the current execution context (for example, request) by storing it in the `PropagationContext`.
 1. If `sample_rand` is missing on an incoming trace, the SDK creates a new one applying **special rules** and uses it for the current execution context:
    1. If `sample_rate` (inside `baggage`) and the sampling decision (trailing `-1` or `-0` from the `sentry-trace` header) are present in the incoming trace, create `sample_rand` so that when compared to the incoming `sample_rate` it would lead to the same sampling decision that is in the `sentry-trace` header. This means, for a decision of `True` generate a random number in half-open range `[0, rate)` and for a decision of `False` generate a random number in range `[rate, 1)`.
    1. If the sampling decision is missing on the incoming trace, generate a random number in range of `[0, 1)` (including 0.0, excluding 1.0), like for a new trace.