Comment 2 for bug 1937354

the new-value argument has to be a string. If they meant "string or nil" then they would not have separately defined "documentation" and "new-value" in the "Arguments and Values"

(setf documentation) new-value x doc-type => new-value.

Arguments and Values:

x---an object.

doc-type---a symbol.

documentation---a string, or nil.

new-value---a string.