split: clarify that --sep-factor is multiplied by the number of f…

…ields to estimate the size of separators/row

[skip ci]

src/cmd/split.rs

@@ -74,7 +74,8 @@ split options:
  This option is mutually exclusive with --size and --chunks.
  --sep-factor <arg> The factor to use when estimating the size of the
  separators (delimiters, quotes & spaces) in the CSV data 
- when splitting by --kb-size. [default: 1.5]
+ when splitting by --kb-size. This is multiplied by the
+ number of fields in the header. [default: 1.5]
 
  -j, --jobs <arg> The number of splitting jobs to run in parallel.
  This only works when the given CSV data has