Manual changes related shock_decomposition

@rattoma Open questions are

1. oo_.realtime_conditional_shock_decomposition is set but not documented
2. The description "conditional shock decomposition math{Y(T|T)} conditional on math{Y(T|T-1)}, i{i.e.} math{Y(t|T)} for math{t=[T-1@dots{}T]}." cannot be right. There is something of with the range of t, I guess
3. I am confused by mixture between the information set and forecast horizon. The realtime_shock_decomposition manual entry always only conditions on T-1, but it takes a general forecast option. It seems there is something missing in the description, most probably with respect to the forecast shock decomposition
4. WriteShockDecomp2Excel uses a file called xlwrite. Is this a file for MAC one needs to install?
5. The pooled is not defined. You refer to plot_shock_decomposition, but there is nothing there
6. I don't understand the part about vintage. It can take values from 0 to 4. But then you repeat the description of realtime, suggesting that the meaning of the four options changes if realtime has a different value