diff --git a/setup/README.md b/setup/README.md
index 49557a85..db41cd08 100644
--- a/setup/README.md
+++ b/setup/README.md
@@ -100,19 +100,20 @@ jobs:
 
 | Name              | Description                                                                                                                                 | Type      | Default     |
 | ----------------- | ------------------------------------------------------------------------------------------------------------------------------------------- | --------- | ----------- |
-| `ghc-version`     | GHC version to use, ex. `9.2` or `9.2.4`.                                                                                                   | string    | `latest`    |
-| `cabal-version`   | Cabal version to use, ex. `3.4`.                                                                                                            | string    | `latest`    |
-| `stack-version`   | Stack version to use, ex. `latest`. Stack will only be installed if `enable-stack` is set.                                                  | string    | `latest`    |
+| `ghc-version`     | GHC version to use, e.g. `9.2` or `9.2.4`.                                                                                                  | `string`  | `latest`    |
+| `cabal-version`   | Cabal version to use, e.g. `3.4`.                                                                                                           | `string`  | `latest`    |
+| `stack-version`   | Stack version to use, e.g. `latest`. Stack will only be installed if `enable-stack` is set.                                                 | `string`  | `latest`    |
 | `enable-stack`    | If set, will setup Stack.                                                                                                                   | "boolean" | false/unset |
 | `stack-no-global` | If set, `enable-stack` must be set. Prevents installing GHC and Cabal globally.                                                             | "boolean" | false/unset |
 | `stack-setup-ghc` | If set, `enable-stack` must be set. Runs stack setup to install the specified GHC. (Note: setting this does _not_ imply `stack-no-global`.) | "boolean" | false/unset |
 | `disable-matcher` | If set, disables match messages from GHC as GitHub CI annotations.                                                                          | "boolean" | false/unset |
-| `cabal-update`    | If set to `false`, skip `cabal update` step.                                                                                                | boolean   | `true`      |
+| `cabal-update`    | If set to `false`, skip `cabal update` step.                                                                                                | `boolean` | `true`      |
 
 Note: "boolean" types are set/unset, not true/false.
 That is, setting any "boolean" to a value other than the empty string (`""`) will be considered true/set.
+However, to avoid confusion and for forward compatibility, it is still recommended to **only use value `true` to set a "boolean" flag.**
 
-In contrast, a proper `boolean` field only accepts values `true` and `false`.
+In contrast, a proper `boolean` input like `cabal-update` only accepts values `true` and `false`.
 
 ## Outputs
 
diff --git a/setup/action.yml b/setup/action.yml
index 02128abe..bc047ec3 100644
--- a/setup/action.yml
+++ b/setup/action.yml
@@ -26,7 +26,10 @@ inputs:
   cabal-update:
     required: false
     default: true
-    description: 'Set to false to prevent `cabal update` from being run.'
+    description: 'Set to `false` to prevent `cabal update` from being run.'
+    # Note: 'cabal-update' only accepts 'true' and 'false' as values.
+    # This is different from the other flags ('enable-stack', 'disable-matcher' etc.)
+    # which are true as soon as they are not null.
   disable-matcher:
     required: false
     description: 'If specified, disables match messages from GHC as GitHub CI annotations.'