doc: add arguments for toKeyValue

  /**

    Generate a key-value-style config file from an attrset.

    *

    * mkKeyValue is the same as in toINI.

    # Inputs

    Structured function argument

    : mkKeyValue (optional, default: `mkKeyValueDefault {} "="`)

      : format a setting line from key and value

    : listsAsDuplicateKeys (optional, default: `false`)

      : allow lists as values for duplicate keys

    : indent (optional, default: `""`)

      : Initial indentation level

  */

  toKeyValue = {

    mkKeyValue ? mkKeyValueDefault {} "=",