Add etymological background to "props". (#177)

content/docs/components-and-props.md

@@ -30,7 +30,7 @@ function Welcome(props) {
 }
 ```
 
-This function is a valid React component because it accepts a single "props" object argument with data and returns a React element. We call such components "functional" because they are literally JavaScript functions.
+This function is a valid React component because it accepts a single "props" (which stands for properties) object argument with data and returns a React element. We call such components "functional" because they are literally JavaScript functions.
 
 You can also use an [ES6 class](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Classes) to define a component: