Improve docstring for Base.Math.exponent (#36438)

Co-authored-by: Stefan Karpinski <stefan@karpinski.org>

Author: igotfr

base/math.jl

@@ -776,9 +776,10 @@ end
 ldexp(x::Float16, q::Integer) = Float16(ldexp(Float32(x), q))
 
 """
-    exponent(x) -> Int
+    exponent(x::AbstractFloat) -> Int
 
 Get the exponent of a normalized floating-point number.
+Returns the largest integer `y` such that `2^y ≤ abs(x)`.
 """
 function exponent(x::T) where T<:IEEEFloat
     @noinline throw1(x) = throw(DomainError(x, "Cannot be NaN or Inf."))