docs: correct MxDataType default-table labels and note non-negative sparse indices

Fix the MxSparseArray type-defaults table in gateway.md to use the real MxDataType
enum member names (Integer, not Integer/LongInteger; Time, not Time/Timestamp) and
clarify the int64 note. Add a docstring sentence to write_array_elements in the
Python client noting that indices and total_length must be non-negative.

clients/python/src/zb_mom_ww_mxgateway/session.py

@@ -516,6 +516,10 @@ class Session:
                 element type of the target array attribute.
             total_length: Total number of elements in the written array.  Must
                 be > 0 and large enough to contain every index in *elements*.
+                Both *total_length* and all keys in *elements* must be
+                non-negative; the gateway rejects negative or out-of-range
+                values with ``InvalidArgument`` (the proto fields are
+                ``uint32``).
             elements: Mapping of zero-based element index to scalar value.
                 Values are converted with :func:`~zb_mom_ww_mxgateway.values.to_mx_value`.
             user_id: Galaxy user id to stamp on the write (requires a prior