Update java sampling documentation page (#4736)

content/en/docs/languages/java/sampling.md

@@ -8,6 +8,60 @@ spans that are generated by a system. Which sampler to use depends on your
 needs. In general, decide which sampler to use at the start of a trace and allow
 the sampling decision to propagate to other services.
 
+## Default behavior
+
+By default, all spans are sampled, resulting in 100% of traces being sampled. If
+your observability backend has constraints or budgetary restrictions on the
+amount of data ingested, you can introduce a sampler and adjust the sample rates
+accordingly. If you do not need to manage data volume, you don't need to set a
+sampler and can use the default.
+
+## Environment variables
+
+You can configure the sampler with environment variables or system properties.
+Reference the [configuration](/docs/languages/java/configuration/) documentation
+for the available options.
+
+For example, to configure the SDK to sample spans such that only 10% of traces
+get created:
+
+```shell
+export OTEL_TRACES_SAMPLER="traceidratio"
+export OTEL_TRACES_SAMPLER_ARG="0.1"
+```
+
+## Samplers
+
+### ParentBasedSampler
+
+When sampling, the ParentBasedSampler is most often used for
+[head sampling](/docs/concepts/sampling/#head-sampling). It uses the sampling
+decision of the Span’s parent, or the fact that there is no parent, to know
+which secondary sampler to use.
+
+If the parent span is sampled, the child span will also be sampled. Conversely,
+if the parent span is not sampled, the child span will not be sampled either.
+This ensures consistency in sampling decisions within a trace.
+
+### TraceIDRatioBasedSampler
+
+The TraceIdRatioBasedSampler deterministically samples a percentage of traces
+that you pass in as a parameter.
+
+This sampler is useful when you want to control the overall sampling rate across
+all traces, regardless of their parent spans. It provides a consistent rate of
+sampling for all traces initiated.
+
+## Configuration in code
+
+{{% alert title="Note" %}} The use of the
+[Java agent](/docs/zero-code/java/agent/),
+[Spring Boot Starter](/docs/zero-code/java/spring-boot-starter/), or
+[SDK autoconfigure](/docs/languages/java/instrumentation/#autoconfiguration) is
+generally recommended for controlling sampling, rather than setting it directly
+in the code. Most users should find the default settings sufficient for their
+needs. {{% /alert %}}
+
 A sampler can be set on the tracer provider using the `setSampler` method, as
 follows:
 
@@ -37,9 +91,9 @@ Other samplers include:
 
 - `traceIdRatioBased`, which samples a fraction of spans, based on the fraction
   given to the sampler. If you set `0.5`, half of all the spans are sampled.
+  Currently, only the ratio of traces that are sampled can be relied on, not how
+  the sampled traces are determined. Only use this sampler for root spans that
+  use `parentBased`.
 - `parentBased`, which uses the parent span to make sampling decisions, if
   present. By default, the tracer provider uses a parentBased sampler with the
   `alwaysOn` sampler.
-
-When in a production environment, consider using the `parentBased` sampler with
-the `traceIdRatioBased` sampler.