Explain the role and rules for `where` row conditions.

where (, any and all) uses the Expr::RowCondition internally, we should probably have a small section on what rules are valid for those. E.g. mention the $it special variable (#401) and that columns of tables can be accessed by their bare name (or the quoted string of their name if they are not valid variable names). Then we can refer to those from the command documentation (@hustcer is there a good way we could allow commands to link to pages in the book?)

Locate the book or command-documentation pages for where and read the references to Expr::RowCondition and issue #401. Add a small section describing valid row-condition rules, including $it and bare or quoted column names, then link to it from the relevant command documentation if the book supports that.