Bugfix: Allow tab to un/mount with transition

Also, fix documentation errors:
- Tab component docs stated that the transition prop of value 'true' would use the 'Fade' transition. Rather, the 'Fade' transition is applied by default. Otherwise, the transition prop accepts 'false' or a react-transition-group 'Transition' component.
- Replace references to v2 with v4

Refs react-bootstrap#3497

src/TabContainer.js

@@ -32,9 +32,9 @@ const propTypes = {
   },
 
   /**
-   * Sets a default animation strategy for all children `<TabPane>`s. Use
-   * `false` to disable, `true` to enable the default `<Fade>` animation or
-   * a react-transition-group v2 `<Transition/>` component.
+   * Sets a default animation strategy for all children `<TabPane>`s.
+   * Defaults to `<Fade>` animation; else, use `false` to disable, or a
+   * custom react-transition-group `<Transition/>` component.
    *
    * @type {{Transition | false}}
    * @default {Fade}

src/TabPane.js

@@ -27,9 +27,9 @@ const propTypes = {
   active: PropTypes.bool,
 
   /**
-   * Use animation when showing or hiding `<TabPane>`s. Use `false` to disable,
-   * `true` to enable the default `<Fade>` animation or
-   * a react-transition-group v2 `<Transition/>` component.
+   * Use animation when showing or hiding `<TabPane>`s. Defaults to `<Fade>`
+   * animation, else use `false` to disable or a react-transition-group
+   * `<Transition/>` component.
    */
   transition: PropTypes.oneOfType([PropTypes.bool, PropTypes.elementType]),
 
@@ -136,7 +136,7 @@ const TabPane = React.forwardRef((props, ref) => {
 
   const prefix = useBootstrapPrefix(bsPrefix, 'tab-pane');
 
-  if (!active && unmountOnExit) return null;
+  if (!active && !Transition && unmountOnExit) return null;
 
   let pane = (
     <Component

src/Tabs.js

@@ -29,9 +29,9 @@ const propTypes = {
   variant: PropTypes.string,
 
   /**
-   * Sets a default animation strategy for all children `<TabPane>`s. Use
-   * `false` to disable, `true` to enable the default `<Fade>` animation or
-   * a react-transition-group v2 `<Transition/>` component.
+   * Sets a default animation strategy for all children `<TabPane>`s.
+   * Defaults to `<Fade>` animation, else use `false` to disable or a
+   * react-transition-group `<Transition/>` component.
    *
    * @type {Transition | false}
    * @default {Fade}