Extend the schema page title to make it clear where to go for the
column/index/projection stuff.
Add an example of using projections to return a subset of values from a
table.
Make the not-yet-supported comment read more naturally.
Add a more complex raw mode example, plus an example showing a projection
combined with raw mode.
It's bad style (IMNSHO), but declare the WT_ITEM locally to make it
clear the type we're using.