docs: clarify JDBC driver source for DataFrameReader.jdbc imports

[sfc-gh-qding]

Note that the JDBC driver JAR must be downloaded from the database vendor and uploaded to a Snowflake stage before being referenced via udtf_configs.imports.

1. Which Jira issue is this PR addressing?

   N/A — documentation-only clarification (docstring change).

2. Fill out the following pre-review checklist:

   • I am adding a new automated test(s) to verify correctness of my new code
     • If this test skips Local Testing mode, I'm requesting review from @snowflakedb/local-testing
   • I am adding new logging messages
   • I am adding a new telemetry message
   • I am adding new credentials
   • I am adding a new dependency
   • If this is a new feature/behavior, I'm adding the Local Testing parity changes.
   • I acknowledge that I have ensured my changes to be thread-safe. Follow the link for more information: Thread-safe Developer Guidelines
   • If adding any arguments to public Snowpark APIs or creating new public Snowpark APIs, I acknowledge that I have ensured my changes include AST support. Follow the link for more information: AST Support Guidelines

3. Please describe how your code solves the related issue.

   Docstring-only change to the udtf_configs.imports parameter description on DataFrameReader.jdbc(). Adds a sentence telling users that the JDBC driver JAR is sourced from the database vendor and uploaded by the user to a Snowflake stage. No code-path change; no AST or thread-safety implications.

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[sfc-gh-qding]

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[sfc-gh-qding]

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