docs(key-exercises): explain variable reassignment in 1-count.js

Describe how the assignment operator evaluates the right-hand side
expression before assigning the result to the count variable

Author: Tarawally

Sprint-1/1-key-exercises/1-count.js

@@ -4,3 +4,6 @@ count = count + 1;
 
 // Line 1 is a variable declaration, creating the count variable with an initial value of 0
 // Describe what line 3 is doing, in particular focus on what = is doing
+
+// Line 3 reassigns the count variable by evaluating the expression on the right hand side of the assignment.
+// The right hand side (count + 1) is evaluated first, then the = operator assigns that value to count.