docs(profiling): Add profile_session_sample_rate docs for python (#12691)

* docs(profiling): Add profile_session_sample_rate docs for python

The newly introduced `profile_session_sample_rate` defaults to `0.0` and needs
to be set to use continuous profiling.

* blurb about the setting itself

Author: Zylphrex

docs/platforms/python/profiling/index.mdx

@@ -59,12 +59,18 @@ Profiling was experimental in SDK versions `1.17.0` and older. Learn how to upgr
 
 <Include name="feature-stage-beta.mdx" />
 
-_(New in version 2.13.0)_
+_(New in version 2.21.0)_
 
 The current profiling implementation stops the profiler automatically after 30 seconds (unless you manually stop it earlier). Naturally, this limitation makes it difficult to get full coverage of your app's execution. We now offer an experimental continuous mode, where profiling data is periodically uploaded while running, with no limit on how long the profiler may run.
 
 To get started with continuous profiling, you can start and stop the profiler directly with `sentry_sdk.profiler.start_profiler` and `sentry_sdk.profiler.stop_profiler`.
 
+### Sampling
+
+Sampling for continuous profiling is determined only once when the SDK is configured. That sampling decision is used to decide if the profiles will be collected or not for the entirety of the process.
+
+Set `profile_session_sample_rate=1.0` to collect continuous profiles for 100% of profile sessions.
+
 <Alert>
 
 If you previously set `profiles_sample_rate` or `profilers_sampler` to use transaction-based profiling, you must remove those lines of code from your configuration in order to use continuous profiling.
@@ -78,6 +84,9 @@ sentry_sdk.init(
     dsn="___PUBLIC_DSN___",
     send_default_pii=True,
     traces_sample_rate=1.0,
+
+    # To collect profiles for all profile sessions, set `profile_session_sample_rate` to 1.0.
+    profile_session_sample_rate=1.0,
 )
 
 sentry_sdk.profiler.start_profiler()