Better variant of difference equation vs. algorithm

Closes modelica#2814

chapters/equations.tex

@@ -3,7 +3,7 @@ \chapter{Equations}\label{equations}
 An \firstuse{equation}\index{equation} is part of a class definition.
 A scalar equation relates scalar variables, i.e., constrains the values that these variables can take simultaneously.
 When $n$-1 variables of an equation containing $n$ variables are known, the value of the $n$th variable can be inferred (solved for).
-In contrast to a statement in an algorithm section, an equation does not define for which of its variable it is to be solved.
+In contrast to an algorithm section, there is no order between the equations in an equation section and they can be solved separately.
 
 \section{Equation Categories}\label{equation-categories}