improves csv docs

docs/website/docs/dlt-ecosystem/destinations/snowflake.md

@@ -152,6 +152,19 @@ When staging is enabled:
 When loading from `parquet`, Snowflake will store `complex` types (JSON) in `VARIANT` as a string. Use the `jsonl` format instead or use `PARSE_JSON` to update the `VARIANT` field after loading.
 :::
 
+### Custom csv formats
+By default we support csv format [produced by our writers](../file-formats/csv.md#default-settings) which is comma delimited, with header and optionally quoted.
+
+You can configure your own formatting ie. when [importing](../../general-usage/resource.md#import-external-files) external `csv` files.
+```toml
+[destination.snowflake.csv_format]
+delimiter="|"
+include_header=false
+on_error_continue=true
+```
+Which will read, `|` delimited file, without header and will continue on errors.
+
+Note that we ignore missing columns `ERROR_ON_COLUMN_COUNT_MISMATCH = FALSE` and we will insert NULL into them.
 
 ## Supported column hints
 Snowflake supports the following [column hints](https://dlthub.com/docs/general-usage/schema#tables-and-columns):

docs/website/docs/dlt-ecosystem/file-formats/csv.md

@@ -28,11 +28,23 @@ info = pipeline.run(some_source(), loader_file_format="csv")
 `dlt` attempts to make both writers to generate similarly looking files
 * separators are commas
 * quotes are **"** and are escaped as **""**
-* `NULL` values are empty strings
+* `NULL` values both are empty strings and empty tokens as in the example below
 * UNIX new lines are used
 * dates are represented as ISO 8601
 * quoting style is "when needed"
 
+Example of NULLs:
+```sh
+text1,text2,text3
+A,B,C
+A,,""
+```
+
+In the last row both `text2` and `text3` values are NULL. Python `csv` writer
+is not able to write unquoted `None` values so we had to settle for `""`
+
+Note: all destinations capable of writing csvs must support it.
+
 ### Change settings
 You can change basic **csv** settings, this may be handy when working with **filesystem** destination. Other destinations are tested
 with standard settings: