docs: fix tuple-prefix range examples

[tclinkenbeard-oai]

Summary

The range examples use a packed null value as an upper bound, but null sorts before other tuple values. That excludes ordinary user keys from the examples' ranges.

• Use the tuple-prefix range helpers in Python, Java, and Go, including the Python city-index example.
• Correct the null-sort comments and clarify that these ranges contain tuples extending the prefix, not the prefix tuple itself.
• Leave the experimental mapped-range mapper and RPC behavior unchanged.

Validation

• A smoke check using the upstream pure-Python tuple encoders from releases 7.1.67, 7.3.77, and 7.4.6 reproduces the original empty ranges and passes with the corrected examples.
• Verified the Java and Go range API signatures in those same release sources.
• Strict MkDocs builds passed for FoundationDB 7.1, 7.3, and 7.4.

No live-cluster test was run; the Java and Go examples were source-checked, not compiled or executed.

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