Remove restriction that sampler description is immutable (#4137)

Fixes #2095

## Changes

State that sampler description can change over time. Provide examples.

---------

Signed-off-by: Yuri Shkuro <github@ysh.us>
Co-authored-by: Robert Pająk <pellared@hotmail.com>

CHANGELOG.md

@@ -11,6 +11,9 @@ release.
 
 ### Traces
 
+- Remove restriction that sampler description is immutable.
+  ([#4137](https://github.com/open-telemetry/opentelemetry-specification/pull/4137))
+
 ### Metrics
 
 ### Logs

specification/trace/sdk.md

@@ -364,7 +364,9 @@ Returns the sampler name or short description with the configuration. This may
 be displayed on debug pages or in the logs. Example:
 `"TraceIdRatioBased{0.000100}"`.
 
-Description MUST NOT change over time and caller can cache the returned value.
+Description MAY change over time, for example, if the sampler supports dynamic
+configuration or otherwise adjusts its parameters.
+Callers SHOULD NOT cache the returned value.
 
 ### Built-in samplers