doc: Update \timing and add LAST_QUERY_MS variable documentation

[kirkw]

This PR adds comprehensive documentation for the LAST_QUERY_MS variable feature and updates the \timing command documentation.

Changes

1. Updated \timing command documentation

• Added clarification that query execution time is always measured regardless of the \timing setting
• Documented that \timing only controls display, not measurement
• Added cross-reference to LAST_QUERY_MS variable

2. Added LAST_QUERY_MS variable documentation

• New section documenting the LAST_QUERY_MS variable
• Explains it contains execution time in milliseconds (3 decimal places)
• States it's always updated after each query execution
• Notes initialization to 0 at session start
• Includes usage example for conditional logic

Files Changed

• doc/src/sgml/ref/psql-ref.sgml: Updated \timing section and added LAST_QUERY_MS variable documentation (+38 lines)

Related

This completes the documentation for the feature implemented in commit 735349f21a6 ("psql: Always measure query timing, store in LAST_QUERY_MS variable").