In clean code book, there is a point that says "Javadocs in Public APIs".
And the same for Effective java book, there is this item :
"Item 56: Write doc comments for all exposed API elements".
So this is my question "Are all public methods considered public APIs?"
A public API is exposed by means of public methods. A public method alone may not be an API. If the the question is if an API can be made of methods in "normal" classes that are not REST points, the answer is yes.