[XML] Document GL_NV_half_float commands that require other extensions

[castholm]

From NV_half_float.txt:

    (added if EXT_fog_coord is supported)

    void FogCoordhNV(half fog);
    void FogCoordhvNV(const half *fog);

    (added if EXT_secondary_color is supported)

    void SecondaryColor3hNV(half red, half green, half blue);
    void SecondaryColor3hvNV(const half *v);

    (added if EXT_vertex_weighting is supported)

    void VertexWeighthNV(half weight);
    void VertexWeighthvNV(const half *weight);

[pdaniell-nv]

@oddhack this is approved to merge if you're happy with it.