add docstring of slerp

src/Quaternion.jl

@@ -385,6 +385,29 @@ function rotationmatrix_normalized(q::Quaternion)
         xz - sy      yz + sx  1 - (xx + yy)]
 end
 
+"""
+    slerp(qa::Quaternion, qb::Quaternion, t::Real)
+
+Spherical linear interpolation (Slerp) between the inputs `qa` and `qb`.
+Since the input is normalized inside the function, the absolute value of the return value will be 1.
+
+# Examples
+```jldoctest
+julia> using Quaternions
+
+julia> qa = Quaternion(1,0,0,0)
+Quaternion{Int64}(1, 0, 0, 0, false)
+
+julia> qb = Quaternion(0,1,0,0)
+Quaternion{Int64}(0, 1, 0, 0, false)
+
+julia> slerp(qa, qb, 0.6)
+QuaternionF64(0.5877852522924731, 0.8090169943749475, 0.0, 0.0, true)
+
+julia> ans ≈ Quaternion(cospi(0.3), sinpi(0.3), 0, 0)
+true
+```
+"""
 @inline function slerp(qa0::Quaternion{T}, qb0::Quaternion{T}, t::T) where T<:Real
     # http://www.euclideanspace.com/maths/algebra/realNormedAlgebra/quaternions/slerp/
     iszero(qa0) && throw(DomainError(qa0, "The input quaternion must be non-zero."))