Confused about using "say" in participle clauses and adverbial participle clauses

I'm using a book to teach my students but in one example it says you can't use "say" in a participle clause because it's not continuous e.g.

• Joanna is a woman who says what she thinks,

not

• ... woman saying what she thinks.

However, in an exercise after the explanation it has this:

• I found a note in red saying "get into the car..."

Can anyone explain why I can use the -ing form in this example?

Solution 1:

What a written document or a sign says is a permanent characteristic of it.

There was a notice saying KEEP OUT.

When we speak of 'what a person always says', we mean their typical form of expression. When Joanna 'says what she thinks', she doesn't always use the same words! As your book says, it is not continuous.