add doc about insert multiple times

https://github.com/4paradigm/OpenMLDB/blob/e2dd0a581ed211056cb764d10c8dcff0007f50e3/docs/zh/quickstart/sdk/java_sdk.md?plain=1#L225

get insert pstmt
for:
pstmt.setxx
pstmt.setxx
pstmt.execute // after execute, cache will be cleaned

ref reuse way in https://github.com/4paradigm/OpenMLDB/blob/e2dd0a581ed211056cb764d10c8dcff0007f50e3/demo/java_quickstart/demo/src/main/java/com/openmldb/demo/App.java#L300

The issue points to a Java SDK documentation file (docs/zh/quickstart/sdk/java_sdk.md) and a demo Java file (demo/java_quickstart/demo/src/main/java/com/openmldb/demo/App.java). First, examine the linked lines to understand the current insert pattern and the proposed reuse method. Then, update the documentation to clarify that after pstmt.execute, the cache is cleaned, and show the reuse pattern from the demo. Verify the changes by reviewing the documentation structure.

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