Minor: Document third argument of date_bin as optional and default …

…value (apache#8912)

* Minor: Document third argument of date_bin as optional

# Rationale
@mhilton  noticed this as part of an internal discussion where we were confused about the output values of `date_bin` for timestamps with timezones (as the default argument is for the unix EPOCH in UTC)

# Changes:
Document the third parameter to `date_bin` better

* prettier

docs/source/user-guide/sql/scalar_functions.md

@@ -1347,7 +1347,8 @@ date_bin(interval, expression, origin-timestamp)
 - **interval**: Bin interval.
 - **expression**: Time expression to operate on.
   Can be a constant, column, or function.
-- **timestamp**: Starting point used to determine bin boundaries.
+- **origin-timestamp**: Optional. Starting point used to determine bin boundaries. If not specified
+  defaults `1970-01-01T00:00:00Z` (the UNIX epoch in UTC).
 
 The following intervals are supported: