fix(develop): clarify trace context's client_sample_rate

When reading the Trace Context docs, I assumed that `client_sample_rate` (like
all the other fields) was expected to be set by the client. Instead, this field
is generated in Relay. If a client sets this field, it will be ignored and
overwritten. Added a note to the docs for this field to clarify how it is meant
to be used.

Author: mjq

develop-docs/sdk/data-model/event-payloads/contexts.mdx

@@ -682,7 +682,7 @@ Additional information that allows Sentry to connect multiple transactions, span
 
 `client_sample_rate`
 
-: _Optional_. The client-side sample rate.
+: _Optional_. The client-side sample rate. This field will be populated by relay. Incoming values are ignored.
 
 - Example: `0.1`