docs: add HIPAA/Zero Data Retention note to variables page

Add section explaining that Vapi does not store variable data when
HIPAA mode or Zero Data Retention is enabled, with link to HIPAA
compliance documentation.

Resolves VAP-11197

Author: vapi-tasker[bot]

fern/assistants/dynamic-variables.mdx

@@ -151,4 +151,18 @@ You are a helpful assistant, talking with a customer via {{transport.conversatio
 - Only ask one question at a time
 - Never use Markdown format; write everything as if it is being said aloud
 {%- endif -%}
-```
+```
+
+## HIPAA and Zero Data Retention mode
+
+When HIPAA mode or Zero Data Retention is enabled for your organization or assistant, Vapi does not store any call data, including variable values. This means:
+
+- Variable values passed via `variableValues` are processed during the call but **not persisted** after the call ends
+- Call logs, recordings, and transcriptions are not stored
+- Variables are only available in real-time during the active call session
+
+This ensures compliance with privacy requirements while still allowing you to personalize conversations with dynamic variables.
+
+<Note>
+For more information on enabling HIPAA compliance and understanding data retention policies, see the [HIPAA Compliance](/security-and-privacy/hipaa) documentation.
+</Note>