Allow star macro to be case insensitive. Improve docs while we're here.

[clrcrl]

This is a:

• new functionality — please ensure the base branch is the latest dev/ branch
• a quality of life change

Description & motivation

This picks up work from #281 (via @mdimercurio) and polishes it off given the more recent changes to this macro. While here, I also improved the docs a little.

Checklist

• I have verified that these changes work locally on the following warehouses (Note: it's okay if you do not have access to all warehouses, this helps us understand what has been covered)
  • BigQuery
  • Postgres
  • Redshift
  • Snowflake
• I have updated the README.md (if applicable)
• I have added tests & descriptions to my models (and macros if applicable)
• I have added an entry to CHANGELOG.md