Clarify mobile replay frame rate is not configurable

[sentry-junior]

Summary

The mobile session replay docs state "Our default frame rate is 1 frame per second", which implies the value is configurable. In practice, frame rate is hardcoded at 1 FPS and is not exposed as a public API on any mobile SDK.

Background

• Android (sentry-java): frameRate field exists in SentryReplayOptions but is marked @ApiStatus.Internal with no public setter — it is intentionally not part of the public API.
• iOS (sentry-cocoa): no frameRate option is surfaced in the public replay config.
• React Native / Flutter: no frame rate config exposed at the SDK layer.

Related upstream issues confirm this is intentional:

• Mobile Session Replay: SDK Recording parameters and API sentry#63248 — frameRate mentioned only as an internal recording parameter
• Feature Request: Reduce Screenshot Capture Rate for Mobile Replays on Low-End Devices sentry#84053 — feature request to expose it was closed as not planned

Scope

Update the mobile session replay docs page to clarify that 1 FPS is a fixed value, not a configurable default. The word "default" implies tunability that doesn't exist.

Implementation

• Change "Our default frame rate is 1 frame per second" to something like "The frame rate is fixed at 1 frame per second and is not currently configurable."
• Optionally remove or qualify the suggestion to open an SDK issue for performance degradation if FPS is the suspected cause.

Action taken on behalf of Simon Zhong.

[linear-code]

DOCS-2602