RFC: label doc (#10275)

* RFC: label doc

* Update functions in doc-utils

commands.md

@@ -271,6 +271,18 @@ With `negate: true`, corresponds to `ifBreak(doc, indent(doc), { groupId })`
 
 It doesn't make sense to apply `indentIfBreak` to the current group because "indent if the current group is broken" is the normal behavior of `indent`. That's why `groupId` is required.
 
+### `label`
+
+_Added in v2.3.0_
+
+```ts
+declare function label(label: string, doc: Doc): Doc;
+```
+
+Mark a doc with a string label. This doesn't affect how the doc is printed, but can be useful for heuristics based on doc introspection.
+
+E.g., to decide how to print an assignment expression, we might want to know whether its right-hand side has been printed as a method call chain, not as a plain function call. If the method chain printing code uses `label` to mark its result, checking that condition can be as easy as `rightHandSideDoc.label === 'method-chain'`.
+
 ### `hardlineWithoutBreakParent` and `literallineWithoutBreakParent`
 
 _Added in v2.3.0_