Update dataset.clj docstring to include :disable-na-as-missing? (#411)

Documents the [recently added](#399) `:disable-na-as-missing?` flag in the `->dataset` options.

Author: kiramclean

src/tech/v3/dataset.clj

@@ -73,6 +73,8 @@
   - `:disable-comment-skipping?` - As default, the `#` character is recognised as a
      line comment when found in the beginning of a line of text in a CSV file,
      and the row will be ignored. Set `true` to disable this behavior.
+  - `:disable-na-as-missing?` - As default, the string \"NA\" is (case-insensitively)
+     parsed as `nil`. Set `false` to disable this behavior.
   - `:max-chars-per-column` - Defaults to 4096.  Columns with more characters that this
      will result in an exception.
   - `:max-num-columns` - Defaults to 8192.  CSV,TSV files with more columns than this