doc(core): add comment on PolynomialSize::log2

tfhe/src/core_crypto/commons/parameters.rs

@@ -137,6 +137,7 @@ pub struct PolynomialSize(pub usize);
 
 impl PolynomialSize {
     /// Return the associated [`PolynomialSizeLog`].
+    /// If the polynomial size is not a power of 2, returns the ceil of its log2
     pub fn log2(&self) -> PolynomialSizeLog {
         PolynomialSizeLog((self.0 as f64).log2().ceil() as usize)
     }