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

processing.org/reference/addassign.html の該当するリファレンスページを開き、+= の文言を確認してください。複合代入式と代入文を、その戻り値も含めて区別するように説明を更新してください。暗黙のキャストについては二次的な詳細として記載してください。