[9.1] [DOCS] Update esql docs readme with 9.x+ version differentiation guidance (#130340)

docs/reference/query-languages/esql/README.md

@@ -105,16 +105,95 @@ To help differentiate between the static and generated content, the generated co
 % This is generated by ESQL's AbstractFunctionTestCase. Do no edit it. See ../README.md for how to regenerate it.
 ```
 
+## Version differentiation in Docs V3
+
+> [!IMPORTANT]
+> Starting with 9.0, we no longer publish separate documentation branches for every minor release (`9.0`, `9.1`, `9.2`, etc.). 
+> This means there won't be a different page for `9.1`, `9.2`, and so on. Instead, all changes landing in subsequent minor releases **will appear on the same page**.
+
+Because we now publish just one docs set off of the `main` branch, we use the [`applies_to` metadata](https://elastic.github.io/docs-builder/syntax/applies/) to differentiate features and their availability across different versions. This is a [cumulative approach](https://elastic.github.io/docs-builder/contribute/#cumulative-docs): instead of creating separate pages for each product and release, we update a **single page** with product- and version-specific details over time.
+
+`applies_to` allows us to clearly communicate when features are introduced, when they transition from preview to GA, and which versions support specific functionality.
+
+This metadata accepts a lifecycle and an optional version.
+
+### Functions and operators
+
+Use the `@FunctionAppliesTo` annotation within the `@FunctionInfo` annotation on function and operator classes to specify the lifecycle and version for functions and operators.
+
+For example, to indicate that a function is in technical preview and applies to version 9.0.0, you would use:
+
+```java
+@FunctionInfo(
+    returnType = "boolean",
+    appliesTo = {
+        @FunctionAppliesTo(lifeCycle = FunctionAppliesToLifecycle.PREVIEW, version = "9.0.0")
+    },
+    ...
+)
+```
+
+When a feature evolves from preview in `9.0` to GA in `9.2`, add a new entry alongside the existing preview entry and remove the `preview = true` boolean:
+
+```java
+@FunctionInfo(
+    returnType = "boolean",
+    preview = false, //  the preview boolean can be removed (or flipped to false) when the function becomes GA
+    appliesTo = {
+        @FunctionAppliesTo(lifeCycle = FunctionAppliesToLifecycle.PREVIEW, version = "9.0.0"),
+        @FunctionAppliesTo(lifeCycle = FunctionAppliesToLifecycle.GA, version = "9.2.0")
+    },
+    ...
+)
+```
+
+We updated [`DocsV3Support.java`](https://github.com/elastic/elasticsearch/blob/main/x-pack/plugin/esql/src/test/java/org/elasticsearch/xpack/esql/expression/function/DocsV3Support.java) to generate the `applies_to` metadata correctly for functions and operators.
+
+### Inline `applies_to` metadata
+
+Use [inline annotations](https://elastic.github.io/docs-builder/syntax/applies/#inline-annotations) to specify `applies_to` metadata in descriptions, parameter lists, etc.
+
+For example, the second item in this list is in technical preview as of version 9.2:
+
+```markdown
+- Item 1
+- Item 2 {applies_to}`stack: preview 9.2.`
+```
+
+### Key rules
+
+1. **Use the `preview = true` boolean** for any tech preview feature - this is required for the Kibana inline docs
+   - **Remove `preview = true`** only when the feature becomes GA on serverless and is _definitely_ going GA in the next minor release
+2. **Never delete `appliesTo` entries** - only add new ones as features evolve from preview to GA
+3. **Use specific versions** (`9.0.0`, `9.1.0`) when known, or just `PREVIEW` without a version if timing is uncertain
+4. **Add `applies_to` to examples** where necessary
+
+> [!IMPORTANT]
+> We don't use `applies_to` in the legacy asciidoc system for 8.x and earlier versions.
+
+### Supported lifecycles
+
+- `PREVIEW` - Feature is in technical preview
+- `GA` - Feature is generally available  
+- `DEPRECATED` - Feature is deprecated and will be removed in a future release
+- `UNAVAILABLE` - Feature is not available in the current version, but may be available in future releases
+
+> [!NOTE] 
+> Unreleased version information is automatically sanitized in the docs build output. For example, say you specify `preview 9.3.0`: 
+> - Before `9.3.0` is released, the live documentation will display "Planned for a future release" instead of the specific version number. 
+>  - This will be updated automatically when the version is released.
+
 ## Tutorials
 
 ### Adding a new command
 
 When adding a new command, for example adding the `CHANGE_POINT` command, do the following:
 1. Create a new file in the `_snippets/commands/layout` directory with the name of the command, for example `change_point.md`.
-2. Add the content for the command to the file. See other files in this directory for examples.
-3. Add the command to the list in `_snippets/lists/processing-commands.md`.
-4. Add an include directive to the `commands/processing-commands.md` file to include the new command.
-5. Add tested examples to the `_snippets/commands/examples` directory. See below for details.
+2. Ensure to specify what versions the command applies to. See [Version differentiation in Docs V3](#version-differentiation-in-docs-v3) for details. [Example PR](https://github.com/elastic/elasticsearch/pull/130314/files#diff-0ab90b6202c5d9eeea75dc95a7cb71dc4d720230342718bff887816771a5a803R3-R6).
+3. Add the content for the command to the file. See other files in this directory for examples.
+4. Add the command to the list in `_snippets/lists/processing-commands.md`.
+5. Add an include directive to the `commands/processing-commands.md` file to include the new command.
+6. Add tested examples to the `_snippets/commands/examples` directory. See below for details.
 
 ### Adding examples to commands