more thorough explanation of input control inference

NEWS.md

@@ -3,7 +3,25 @@ rmarkdown 2.29
 
 - `find_external_resources()` now correctly detects knitr child document provided with option like `child = c("child.Rmd")` (thanks, @rempsyc, #2574).
 
-- A parameter allowing multiple selections uses a select input control by default regardless of the number of choices.
+- `knit_params_ask()` uses a `select` input for parameters which allow multiple selected values. Previously, a `radio` input was incorrectly used when the parameter had a small number of choices.
+
+    ```yaml
+    params:
+        primaries:
+            choices: ["red", "yellow", "blue"]
+            multiple: true
+    ```
+    
+    When `multiple` is not enabled, parameter configuration still uses `radio` when there are fewer than five choices.
+
+    The `input` parameter field can still be used to force the configuration control.
+
+    ```yaml
+    params:
+        grade:
+            input: radio
+            choices: ["A", "B", "C", "D", "F"]
+    ```
 
 
 rmarkdown 2.28