Compound assignment expressions described as being equivalent to statements

Issue description

The reference has the following to say on += :

Combines addition with assignment. The expression a+=b is equivalent to a=a+b.

The biggest problem is that the expression a+=b has a return value (namely a+b), so it
can be used as a part of a larger expression. The statement a=a+b has no return value
and can not be used as part of a larger expression>

As a smaller problem a+=b involves an implicit cast, but that belongs to the detail department.

For students learning to program, it is important to be clear about the distinction between
expressions and statements, so I suggest a rewording that makes it clear that a+=b
is an expression.

URL(s) of affected page(s)

https://processing.org/reference/addassign.html

Proposed fix

打开 processing.org/reference/addassign.html 中受影响的参考页面，检查关于 += 的措辞。更新说明，使其区分复合赋值表达式和赋值语句，包括其返回值；将隐式转换作为次要细节说明。