Clarify documentation on price projections

[martinstringer]

It would be great for new users if the docs could be really clear that the price projections file:

• does not need to contain all commodities
• commodities not included in this file will be assigned a price of zero for all years, unless...
• the commodity is created inside the simulation, in which case the price in the projections file is not used.

[martinstringer]

Have made some tiny tweaks (in linked pull request) that would have been enough to put things beyond doubt for me when I looked this up. However, I may be wrong in my understanding of the behaviour or there may be a better way to explain so please feel free to improve!