Change erroneous documentation for .flex-fill (#27265)

The current documentation for .flex-fill indicated that the flex
items would have equal widths, regardless of content. This update
ensures that the documentation reflects the fact that the width
of the flex items depends on their actual content.

site/docs/4.1/utilities/flex.md

@@ -219,11 +219,11 @@ Responsive variations also exist for `align-self`.
 
 ## Fill
 
-Use the `.flex-fill` class on a series of sibling elements to force them into equal widths while taking up all available horizontal space. [Especially useful for equal-width, or justified, navigation]({{ site.baseurl }}/docs/{{ site.docs_version }}/components/navs/#working-with-flex-utilities).
+Use the `.flex-fill` class on a series of sibling elements to force them into widths equal to their content (or equal widths if their content does not surpass their border-boxes) while taking up all available horizontal space. 
 
 {% capture example %}
 <div class="d-flex bd-highlight">
-  <div class="p-2 flex-fill bd-highlight">Flex item</div>
+  <div class="p-2 flex-fill bd-highlight">Flex item with a lot of content</div>
   <div class="p-2 flex-fill bd-highlight">Flex item</div>
   <div class="p-2 flex-fill bd-highlight">Flex item</div>
 </div>