Document description, default and options keys for workflow_dispatch inputs

The workflow-syntax reference documented only required and type for
on.workflow_dispatch.inputs.<input_id>, but the example on the same
page uses description, default and options too, and the workflow
schema defines all of them. Adds a section for each of the three
missing keys, matching the style of the existing entries.

Author: ssmurfgg04-gif

content/actions/reference/workflows-and-actions/workflow-syntax.md

@@ -329,14 +329,26 @@ A boolean specifying whether the secret must be supplied.
 
 {% data reusables.actions.workflow-dispatch-inputs-example %}
 
+## `on.workflow_dispatch.inputs.<input_id>.description`
+
+A string description of the input parameter.
+
 ## `on.workflow_dispatch.inputs.<input_id>.required`
 
 A boolean specifying whether the input must be supplied.
 
+## `on.workflow_dispatch.inputs.<input_id>.default`
+
+The default value for this input, used when the workflow is triggered manually without a value being supplied for the input.
+
 ## `on.workflow_dispatch.inputs.<input_id>.type`
 
 The value of this parameter is a string specifying the data type of the input. This must be one of: `boolean`, `choice`, `number`, `environment` or `string`.
 
+## `on.workflow_dispatch.inputs.<input_id>.options`
+
+A list of strings containing the options shown in the dropdown list for this input when the workflow is triggered manually. This can only be used when the input's `type` is `choice`.
+
 ## `permissions`
 
 {% data reusables.actions.jobs.section-assigning-permissions-to-jobs %}