docs: improve documentation of prep_techmix

[jdhoffa]

Indeed, it should be checking for presence of "projected" value in metric column. We missed that. The benchmark is optional. I propose to reword it in the documentation to:

• The column metric must have a "projected" value and
  a single scenario (e.g. "target_sds").
• (Optional) The column metric may contain a benchmark
  (e.g. "corporate_economy").

Originally posted by @MonikaFu in #552 (comment)