count: document three count modes

[skip ci]

src/cmd/count.rs

@@ -1,7 +1,18 @@
 static USAGE: &str = r#"
-Prints a count of the number of records in the CSV data. If the polars feature is
-enabled, it will use the much faster multithreaded, mem-mapped Polars CSV reader.
-Otherwise, it will use the regular, single-threaded CSV reader.
+Returns a count of the number of records in the CSV data.
+
+It has three modes of operation:
+ 1. If a valid index is present, it will use it to lookup the count and
+ return instantaneously. (fastest)
+
+ If no index is present, it will read the CSV and count the number
+ of records by scanning the file. 
+ 
+ 2. If the polars feature is enabled, it will use the multithreaded,
+ mem-mapped Polars CSV reader. (faster - not available on qsvlite)
+ 
+ 3. If the polars feature is not enabled, it will use the "regular",
+ single-threaded CSV reader.
 
 Note that the count will not include the header row (unless --no-headers is
 given).
@@ -18,15 +29,18 @@ count options:
  Its an estimate as it doesn't count quotes, and will be an
  undercount if the record has quoted fields.
  The count and width are separated by a semicolon.
+ Note that this option will require scanning the entire file
+ using the "regular", single-threaded, streaming CSV reader
+ and will not use the index nor the Polars CSV reader.
 
  WHEN THE POLARS FEATURE IS ENABLED:
  --no-polars Use the "regular", single-threaded, streaming CSV reader instead
  of the much faster multithreaded, mem-mapped Polars CSV reader.
  Use this when you encounter memory issues when counting with the
  Polars CSV reader. The streaming reader is slower but can read
  any valid CSV file of any size.
- --low-memory Use the Polars CSV Reader's low-memory mode. This
- mode is slower but uses less memory. If counting still fails,
+ --low-memory Use the Polars CSV Reader's low-memory mode. This mode
+ is slower but uses less memory. If counting still fails,
  use --no-polars instead to use the streaming CSV reader.