update sort docs

Author: aljazerzen

web/book/src/transforms/sort.md

@@ -90,6 +90,32 @@ it).
 
 ## Nulls
 
+At the moment, PRQL does not guarantee any position of null values when
+ordering. To be precise, they are not comparable to any other values, which
+means that an ordered array may contain nulls first, last, or at any position in
+between.
+
+However, prql-compiler will emit `NULLS LAST` (or dialect equivalent) in all
+`ORDER BY` clauses, with intention to make query results deterministic across
+different engines. This behavior may change, see
+[issue #2622](https://github.com/PRQL/prql/issues/).
+
+To enforce `NULLS FIRST` or `NULLS LAST` right now, the following workaround can
+be used:
+
+```prql
+from artists
+sort {artist_id != null, artist_id} # nulls first
+```
+
+```prql
+from artists
+sort {artist_id == null, artist_id} # nulls last
+```
+
+<!--
+TODO:
+
 PRQL defaults to `NULLS LAST` when compiling to SQL. Because databases have
 different defaults, the compiler emits this for all targets for which it's not a
 default{{footnote: except for MSSQL, which don't support this}}.
@@ -113,4 +139,4 @@ take 42
 
 ```admonish info
 Check out [DuckDB #7174](https://github.com/duckdb/duckdb/pull/7174) for a survey of various databases' implementations.
-```
+``` -->