luau: improve usage text

call out two main subcommands

[skip ci]

src/cmd/luau.rs

@@ -10,11 +10,16 @@ The executed Luau has 3 ways to reference row columns (as strings):
  1. Directly by using column name (e.g. Amount), can be disabled with -g
  2. Indexing col variable by column name: col.Amount or col["Total Balance"]
  3. Indexing col variable by column 1-based index: col[1], col[2], etc.
- This is only available with the --colindex and --no-headers options.
+ This is only available with the --colindex or --no-headers options.
 
 Of course, if your input has no headers, then 3. will be the only available
 option.
 
+It has two subcommands:
+ map - Create new columns by mapping the result of a Luau script for each row.
+ filter - Filter rows by executing a Luau script for each row. Rows that return
+ true are kept, the rest are filtered out.
+
 Some usage examples:
 
  Sum numeric columns 'a' and 'b' and call new column 'c'