docs: Add survey abandoned event to Surveys documentation (#15306)

* Add `survey abandoned` event to survey lifecycle events docs

* docs: add `survey abandoned` event to JS SDK survey events documentation

* docs: add survey abandoned event to partial survey responses section

* docs: add survey abandoned event to viewing results event list

* revert: remove survey abandoned additions from implementing-custom-surveys.mdx

* revert: remove survey abandoned additions from JS SDK usage docs

* Apply suggestions from code review

---------

Co-authored-by: inkeep[bot] <257615677+inkeep[bot]@users.noreply.github.com>
Co-authored-by: Adam Bowker <adam.b@posthog.com>

Author: inkeep[bot]

contents/docs/surveys/creating-surveys.mdx

@@ -399,3 +399,8 @@ All `survey sent` events triggered during a single user's attempt to complete a
 
 **Billing for partial responses:**
 We charge based on unique survey submissions, not individual `survey sent` events. Therefore, you are charged only once per unique `$survey_submission_id`. For example, if your survey has 5 questions and a user answers 3 of them (triggering multiple `survey sent` events with `$survey_completed: false`), you will still be charged for only one survey response, as all those events will share the same `$survey_submission_id`.
+
+**The `survey abandoned` event:**
+When a user answers at least one question but navigates away or closes the page before completing all questions, PostHog automatically captures a `survey abandoned` event. This event includes the `$survey_id` property identifying the survey. It's different from `survey dismissed`, which fires when a user explicitly closes the survey without answering.
+
+You can use the `survey abandoned` event in [insights](/docs/surveys/viewing-results#2-creating-your-own-insights) to measure drop-off rates and identify which surveys users start but don't finish.

contents/docs/surveys/viewing-results.mdx

@@ -42,11 +42,12 @@ For rating questions (including NPS), the trend charts include an **Open as new
 
 To create insights from survey results, navigate to the [insights tab](https://app.posthog.com/insights) and click on `New insights` button.
 
-There are 3 survey-related events you create insights from:
+There are four survey-related events you can create insights from:
 
-1. `survey dismissed`
-2. `survey shown`
-3. `survey sent` (for responses)
+1. `survey shown` - fires when a survey is displayed to a user
+2. `survey sent` - fires when a user responds to a question
+3. `survey dismissed` - fires when a user explicitly closes the survey
+4. `survey abandoned` - (for surveys with partial completion enabled) fires when a user navigates away from the page before completing the survey
 
 When creating an insight, select any of these events. To see data for a specific survey, create a filter using the `Survey ID` or `Survey Name` properties.