Description of tranching in docs doesn't match what we do

[alexdewar]

The documentation in investment.md talks about tranching using load factor, which is not what we currently do. There is a misleading note which says that we don't do tranching at all, when actually we do tranching based on only considering part of an asset's potential capacity at once (determined by the capacity_limit_factor parameter).

We should make the documentation clearer. Note that we may change our approach to tranching (it may be causing issues).

See discussion: #1125 (comment)