Add docstring

Author: jishnub

src/special.jl

@@ -320,6 +320,26 @@ _diag_or_value(A::Diagonal) = A.diag
 _diag_or_value(A::UniformScaling) = A.λ
 
 # fill[stored]! methods
+"""
+    fillstored!(A::AbstractMatrix, x)
+
+Fill only the stored indices of a structured matrix `A` with the value `x`.
+
+# Example
+```jldoctest
+julia> A = Tridiagonal(zeros(2), zeros(3), zeros(2))
+3×3 Tridiagonal{Float64, Vector{Float64}}:
+ 0.0  0.0   ⋅
+ 0.0  0.0  0.0
+  ⋅   0.0  0.0
+
+julia> LinearAlgebra.fillstored!(A, 2)
+3×3 Tridiagonal{Float64, Vector{Float64}}:
+ 2.0  2.0   ⋅
+ 2.0  2.0  2.0
+  ⋅   2.0  2.0
+```
+"""
 fillstored!(A::Diagonal, x) = (fill!(A.diag, x); A)
 fillstored!(A::Bidiagonal, x) = (fill!(A.dv, x); fill!(A.ev, x); A)
 fillstored!(A::Tridiagonal, x) = (fill!(A.dl, x); fill!(A.d, x); fill!(A.du, x); A)