Improve documentation of share constraints

[glatterf42]

What is this about?

As discovered through #875, our documentation for share constraints is lacking. For starters, map_shares_commodity_share and map_shares_commodity_total are missing in the documentation. They are mentioned here, but not explained: what can you do with them, what do their index sets mean? In particular, what does node_share mean and how does it contrast with node? And by defining these in the map_*s, why does e.g. share_commodity_lo need another node_share dimension?

In addition, the index dimensions listed for share_* parameters are erroneous: they reference node_share, node, and node_loc, while the GAMS codes calls all of them node_share. This should be fixed here.