[css-transitions-1][editorial] Add the word gradual to clarify that it's happening over time

Author: fantasai

css-transitions-1/Overview.bs

@@ -141,7 +141,7 @@ Value Definitions {#values}
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-        Normally when the value of a CSS property changes, the rendered result is instantly updated, with the affected elements immediately changing from the old property value to the new property value. This section describes a way to specify transitions using new CSS properties. These properties are used to animate smoothly from the old state to the new state over time.
+        Normally when the value of a CSS property changes, the rendered result is instantly updated, with the affected elements immediately changing from the old property value to the new property value. This section describes a way to specify gradual transitions using new CSS properties. These properties are used to animate smoothly from the old state to the new state over time.
       
       
         For example, suppose that transitions of one second have been defined on the 'left' and