Add note to EIGEN_DONT_PARALLELIZE into preprocessor documentation page (requested in IRC)
Add note to EIGEN_DONT_PARALLELIZE into preprocessor documentation page (requested in IRC)

ritsu1228/eigen,toastedcrumpets/eigen,Zefz/eigen,ROCmSoftwarePlatform/hipeigen,Zefz/eigen,ROCmSoftwarePlatform/hipeigen,TSC21/Eigen,pasuka/eigen,TSC21/Eigen,pasuka/eigen,ritsu1228/eigen,ritsu1228/eigen,pasuka/eigen,ritsu1228/eigen,ROCmSoftwarePlatform/hipeigen,toastedcrumpets/eigen,toastedcrumpets/eigen,TSC21/Eigen,Zefz/eigen,toastedcrumpets/eigen,ROCmSoftwarePlatform/hipeigen,Zefz/eigen,ritsu1228/eigen,pasuka/eigen,TSC21/Eigen,pasuka/eigen