Document limitations of the arrow_integration_test crate (#8738)

# Which issue does this PR close?

- Closes #8684

# Rationale for this change

The crate is apparently only intended for internal use. However, the
crate documentation does not mention this anywhere. Given that the crate
is severely incomplete in certain areas, we should document this
clearly.

# What changes are included in this PR?

- add a disclaimer to the crate-level docs that the crate is only
intended for integration-testing the Arrow project itself and not
recommended for outside use
- Document that `ArrowJsonBatch::from_batch` is deliberately incomplete
and only supports `Int8` arrays right now

# Are these changes tested?

doc-only change

# Are there any user-facing changes?

yes, the mentioned doc changes

Author: phil-opp

arrow-integration-test/src/lib.rs

@@ -15,11 +15,19 @@
 // specific language governing permissions and limitations
 // under the License.
 
-//! Support for the [Apache Arrow JSON test data format](https://github.com/apache/arrow/blob/master/docs/source/format/Integration.rst#json-test-data-format)
+//! Partial support for the [Apache Arrow JSON test data format](https://github.com/apache/arrow/blob/master/docs/source/format/Integration.rst#json-test-data-format)
 //!
 //! These utilities define structs that read the integration JSON format for integration testing purposes.
 //!
 //! This is not a canonical format, but provides a human-readable way of verifying language implementations
+//!
+//! <div class="warning">
+//!
+//! This crate is **only intended for integration testing the
+//! [Arrow project](https://github.com/apache/arrow-rs)**. It is not [intended for usage outside of
+//! this context](https://github.com/apache/arrow-rs/issues/8684#issuecomment-3433193158).
+//!
+//! </div>
 
 #![doc(
     html_logo_url = "https://arrow.apache.org/img/arrow-logo_chevrons_black-txt_white-bg.svg",
@@ -1043,6 +1051,16 @@ fn create_null_buf(json_col: &ArrowJsonColumn) -> Buffer {
 
 impl ArrowJsonBatch {
     /// Convert a [`RecordBatch`] to an [`ArrowJsonBatch`]
+    ///
+    /// <div class="warning">
+    ///
+    /// This function is **deliberately incomplete**! As noted in the crate-level documentation,
+    /// this crate is only intended for use within the Arrow project itself.
+    ///
+    /// Right now, this function only supports `DataType::Int8` columns. Other data types will lead
+    /// to an empty `ArrowJsonColumn`.
+    ///
+    /// </div>
     pub fn from_batch(batch: &RecordBatch) -> ArrowJsonBatch {
         let mut json_batch = ArrowJsonBatch {
             count: batch.num_rows(),