Updates to Tabs documentation (#506)

When implementing this I initially didn't spot that the the tab slots
would accept a block. Feel like this is probably the default use case
(as tabs normally contain a fair amount of content) so worth
highlighting this a bit more.

Also added some documentation on the `title` argument, and renamed the
local variable from `c` to `tabs`.

guide/content/components/tabs.slim

@@ -7,13 +7,15 @@ p
     displaying one section at a time.
 
 == render('/partials/example.*',
-  caption: "Tabs with text and block content",
+  caption: "How to use the component",
   code: tabs_normal,
   render_in_boxes: true) do
 
   markdown:
-    The tabs component can contain one or more `tab` slots. Each tab requires a
-    `label` which will form the link at the top, and the content can be set
-    either using the `text` argument or via a block of HTML.
+    The tabs component must be given a `title`, which is displayed above a list of jump links on narrow screens or if javascript is unavailable. This is usually set to ‘Contents’.
+
+    Each tab requires a `label` which will form the link at the top.
+
+    The content can be set by using either a block of HTML, or the `text` argument.
 
 == render('/partials/related-navigation.*', links: tabs_info)

guide/lib/examples/tabs_helpers.rb

@@ -2,11 +2,12 @@ module Examples
   module TabsHelpers
     def tabs_normal
       <<~TABS
-        = govuk_tabs(title: "Monday’s child nursery rhyme") do |c|
-          - c.with_tab(label: "Monday", text: "Monday’s child is fair of face")
-          - c.with_tab(label: "Tuesday", text: "Tuesday’s child is full of grace")
-          - c.with_tab(label: "Wednesday")
-            | Wednesday’s child is full of woe
+        = govuk_tabs(title: "Contents") do |tabs|
+          - tabs.with_tab(label: "Text", text: "This was set using a text argument")
+          - tabs.with_tab(label: "Inline block") { "This was set using an inline block of content" }
+          - tabs.with_tab(label: "Regular block") do
+            p This was set using a block of HTML.
+            p Use this style if you need complex markup within your tab.
       TABS
     end
   end