Update double transpose note in docs (#214)

Fix #82

docs/src/index.md

@@ -35,10 +35,13 @@ one place over.
 All operations on sparse matrices are carefully implemented to exploit the CSC data structure
 for performance, and to avoid expensive operations.
 
-If you have data in CSC format from a different application or library, and wish to import it
-in Julia, make sure that you use 1-based indexing. The row indices in every column need to be
-sorted. If your `SparseMatrixCSC` object contains unsorted row indices, one quick way to sort
-them is by doing a double transpose.
+If you have data in CSC format from a different application or
+library, and wish to import it in Julia, make sure that you use
+1-based indexing. The row indices in every column need to be sorted,
+and if they are not, the matrix will display incorrectly.  If your
+`SparseMatrixCSC` object contains unsorted row indices, one quick way
+to sort them is by doing a double transpose. Since the transpose operation
+is lazy, make a copy to materialize each transpose.
 
 In some applications, it is convenient to store explicit zero values in a `SparseMatrixCSC`. These
 *are* accepted by functions in `Base` (but there is no guarantee that they will be preserved in
@@ -242,7 +245,7 @@ Several other Julia packages provide sparse matrix implementations that should b
 
 1. [SuiteSparseGraphBLAS.jl](https://github.com/JuliaSparse/SuiteSparseGraphBLAS.jl) is a wrapper over the fast, multithreaded SuiteSparse:GraphBLAS C library. On CPU this is typically the fastest option, often significantly outperforming MKLSparse.
 
-2. [CUDA.jl](https://github.com/JuliaGPU/CUDA.jl) exposes the [CUSPARSE](https://docs.nvidia.com/cuda/cusparse/index.html) library for GPU sparse matrix operations. 
+2. [CUDA.jl](https://github.com/JuliaGPU/CUDA.jl) exposes the [CUSPARSE](https://docs.nvidia.com/cuda/cusparse/index.html) library for GPU sparse matrix operations.
 
 3. [SparseMatricesCSR.jl](https://github.com/gridap/SparseMatricesCSR.jl) provides a Julia native implementation of the Compressed Sparse Rows (CSR) format.