Update fluid heading styles docs (carbon-design-system#4043)

* Update fluid heading styles docs

* Update src/pages/elements/typography/type-sets.mdx

Co-authored-by: Alison Joseph <alisonejoseph@Gmail.com>

* Update copy

---------

Co-authored-by: Alison Joseph <alisonejoseph@Gmail.com>

Author: abdonrd

src/pages/elements/typography/type-sets.mdx

@@ -111,13 +111,17 @@ headings are needed.
 
 The fluid heading styles are primarily used in web pages, and are therefore part
 of the expressive set of type styles. These headings are responsive and the type
-styles change size at different breakpoints.
+styles actually change size incrementally (almost imperceptibly) between the
+different breakpoints — hence the name "fluid."
 
 Do not use these styles inside a container. They may be used in product pages
 where text sits outside of a container, and a blend of expressive and productive
 type styles is desired for hierarchy and distinction. For more information, see
 [Style strategies](/guidelines/typography/style-strategies).
 
+_Note: the slider below shows the type sizes jumping abruptly between
+breakpoints, which is not a good representation of the actual behavior._
+
 <TypesetStyle typesets="fluidHeadings" breakpointControls={true} />
 
 ## Fluid display styles