Document AZURE_TRACING_GEN_AI_CONTENT_RECORDING_ENABLED env var in azure-ai-inference

[lmolkova]

See

azure-sdk-for-java/sdk/ai/azure-ai-inference/src/samples/java/com/azure/ai/inference/usage/TraceChatCompletionsToolCallSample.java

Line 118 in bff7b81

// uncomment to capture message content in the telemetry (or you can set AZURE_TRACING_GEN_AI_CONTENT_RECORDING_ENABLED environment variable to `true`)

- there is no other documentation, we should do something similar to python where we document it in readme and in each example in setup comments