Often times textures are broken down int many different "maps", which provide different information about the object and how it should look. At the bare minimum, three colors are passed in, and the 4th component alpha may or may not be used. In addition to that, color maps that represent normals to be used when calculating specular highlights and/or bump mapping is often provided in a separate texture. Rather than generate a totally separate map, this hint is simply providing a less detailed, but more efficient way to pass specular highlight information to the shading pipeline.
This hint is indicating that you can save memory by integrating your specular map information into the 4th component of your diffuse color. When your vertex data gets to the pixel shader, without any additional computations performed you will have a 4 component color vector available to you. The 4th component (typically used for alpha) can be used for other information, and in this situation is recommended to be used for specular highlights instead of alpha.