docs: include NestedLoopJoinExec metrics in EXPLAIN ANALYZE guide (issue #19045)

[muien5080]

This update adds documentation for NestedLoopJoinExec to the EXPLAIN ANALYZE / metrics user guide. The operator recently gained a selectivity metric, and this PR documents its behavior along with the other join operators.

The new section explains:

• What NestedLoopJoinExec is used for (non-equijoins and cases where hash or sort-merge joins cannot be applied)
• The metrics it reports under EXPLAIN ANALYZE
• The meaning of the new selectivity metric (output_rows / input_rows)
• An example output fragment from an EXPLAIN ANALYZE plan

This helps users interpret nested-loop join behavior and understand join selectivity during query analysis.

Related: apache/datafusion#19045

[2010YOUY01]

The metrics don’t seem to match the actual ones, is this PR AI-generated? If so, FYI: https://datafusion.apache.org/contributor-guide/index.html#ai-assisted-contributions