Updated P&S explainer with privacy text

Addressed private browsing modes (resolves immersive-web#750)

privacy-security-explainer.md

@@ -5,6 +5,11 @@ The WebXR Device API enables developers to build content for AR and VR hardware
 ### Sensitive information
 In the context of XR, sensitive information includes, but is not limited to, user configurable data such as interpupillary distance (IPD) and sensor-based data such as poses. All `immersive` sessions will expose some amount of sensitive data, due to the user's pose being necessary to render anything. However, in some cases, the same sensitive information will also be exposed via `inline` sessions. 
 
+### Private Browsing modes
+User agents may support a mode (e.g., private browsing) of operation intended to preserve user anonymity and/or ensure records of browsing activity are not persisted on the client.
+
+There is no persistent data or unique user identifier data generated by the WebXR Device API. As such, there are no additional requirements for such modes.
+
 # Protection types
 WebXR must be structured to ensure end users are protected from developers gathering and using sensitive information inappropriately. The necessary protections will vary based on the sensitive data being guarded, and, in some cases, more than one protection is necessary to adequately address the potential threats exposed by specific sensitive information.
 
@@ -275,4 +280,4 @@ In response, the UA must ensure that:
 * Any group of `local`, `local-floor`, and `bounded-floor` reference spaces that are capable of being related to one another must share a common native origin; this restriction does not apply when `unbounded` reference spaces are also permitted to be created
 * All `XRPose` and `XRViewerPose` 6DoF pose data computed using a `local` reference space is [limited](#limiting) to a reasonable distance from the reference space's native origin; the suggested default distance is 15 meters in each direction
 
-If these requirements are not met, the promise returned from `XRSession.requestReferenceSpace()` must be rejected.
+If these requirements are not met, the promise returned from `XRSession.requestReferenceSpace()` must be rejected.