Is it true that a "which" clause should always start with a comma?

Solution 1:

No, it is not true. This is a bug in Microsoft Word, just one out of millions. You really should not take it too seriously.

What is correct is that if there is no comma before the which, that it is a defining relative clause, but if there is a comma, it is a non-defining one.

1. The rats which are the snake's food are growing crazily. (defining)
2. The rats, which are the snake's food, are growing crazily. (non-defining)
3. The rats that are the snake's food are growing crazily. (defining)
4. The rats (which are the snake's food) are growing crazily. (non-defining)
5. The rats are growing crazily.

Numbers 1 and 3 mean exactly the same thing.

Numbers 2 and 4 mean the same thing as each other, and are essentially the same as number 5.

This is discussed in this answer to ELU question #96.