docs(sql): improve Phase 4 Doxygen documentation clarity

Address code review recommendations to improve documentation accuracy
and add safety warnings for DDL-executing functions.

**Documentation Improvements:**

1. **src/encryptindex/functions.sql:102-103** - Clarified select_target_columns
   - Enhanced note to explain full JOIN logic (checks both column_name
     and column_name_encrypted with type verification)
   - Previous: "LEFT JOIN returns NULL when no match"
   - Updated: Explains both name matching variants and type requirement

2. **src/jsonb/functions.sql:296** - Enhanced jsonb_array_length exception
   - More specific @throws documentation with exact error message
   - Previous: "if value is not an array (missing 'a' flag)"
   - Updated: "'cannot get array length of a non-array' if 'a' flag
     is missing or not true"

3. **src/jsonb/functions.sql:29** - Clarified NULL handling in jsonb_path_query
   - Precise SETOF return semantics
   - Previous: "Returns NULL if val is NULL"
   - Updated: "Returns a set containing NULL if val is NULL"

**Safety Warnings:**

4. **src/encryptindex/functions.sql:152** - Added @warning to create_encrypted_columns
   - Highlights dynamic DDL execution (ALTER TABLE ADD COLUMN)
   - Alerts users to schema modification side effects

5. **src/encryptindex/functions.sql:180** - Added @warning to rename_encrypted_columns
   - Highlights dynamic DDL execution (ALTER TABLE RENAME COLUMN)
   - Alerts users to schema modification side effects

**Quality Assurance:**
- ✅ All tests passing (59 test files)
- ✅ Code review approved (CODE_REVIEW_PHASE_4_COMPLETE.md)
- ✅ 0 blocking issues
- ✅ All NON-BLOCKING recommendations addressed

**Note:** User fixed blocking code bug in jsonb_path_query_first separately

Author: tobyhede