doc: not about cross-platfrom build of jni-tests
ref
* Building
-1. You need environment variable JAVA_HOME in such way that
+1. Cross-platform build is not supported
+2. You need environment variable JAVA_HOME in such way that
$JAVA_HOME/include/jni.h exists
-2. cargo build
-3. run tests via `cd .. && python ci_build_and_test.py --java-only-tests`
+3. cargo build
+4. run tests via `cd .. && python ci_build_and_test.py --java-only-tests`
