docs(plugin-api): fix extra_small_font size, cache metric key and scroll pacing example

- PLUGIN_API_REFERENCE: extra_small_font loads at 7, not 6 (crisp_size
  snaps it, src/display_manager.py); get_cache_metrics() returns
  cache_hit_rate, not hit_rate (src/cache/cache_metrics.py).
- ADVANCED_PLUGIN_DEVELOPMENT: the basic scrolling example slept in a loop
  and never passed frame_hold; use ScrollHelper + scroll_config.configure()
  and set_scrolling_state(True, frame_hold=...) as PLUGIN_API_REFERENCE does.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/PLUGIN_API_REFERENCE.md

@@ -620,7 +620,7 @@ The Display Manager provides several pre-loaded fonts:
 display_manager.regular_font      # Press Start 2P, size 8
 display_manager.small_font        # Press Start 2P, size 8
 display_manager.calendar_font     # 5x7 BDF font
-display_manager.extra_small_font  # 4x6 TTF font, size 6
+display_manager.extra_small_font  # 4x6 TTF font, size 7 (6 snapped to its pixel grid)
 display_manager.bdf_5x7_font     # Alias for calendar_font
 ```
 
@@ -854,12 +854,12 @@ for file_info in files:
 
 Get cache performance metrics.
 
-**Returns**: Dictionary with cache statistics (hits, misses, hit rate, etc.)
+**Returns**: Dictionary with cache statistics (`total_requests`, `cache_hit_rate`, `background_hit_rate`, `api_calls_saved`, `average_fetch_time`, etc.)
 
 **Example**:
 ```python
 metrics = self.cache_manager.get_cache_metrics()
-self.logger.info(f"Cache hit rate: {metrics['hit_rate']:.2%}")
+self.logger.info(f"Cache hit rate: {metrics['cache_hit_rate']:.2%}")
 ```
 
 #### `get_memory_cache_stats() -> Dict[str, Any]`