Documentation for `settings.toml`: options for `demand_share` are out of date

[tsmbland]

The documentation for settings.toml (here) lists the options for demand_share as:

• new_and_retro
• market_demand

According to the code, the valid options are actually:

• new_and_retro (or default)
• standard_demand
• unmet_demand

The documentation needs to be updated, including descriptions for each of the new options