Review feasible_ddp documentation

So far I have detected these two problems:

• Q_x and Q_u missed the L_x and L_u terms, respectively. (section 1.5.2)
• k and K have an error in its formula, it should be k=Q_{uu}^{-1}Q_u and K=Q_{uu}^{-1}Q_{ux} (section 1.5.2)
• few extra typos