docs: document multi-page config-settings (sponge + paths)

- §1 Scrollback History: __fish_scrollback_history_* source-of-truth + exported
  mirror, and the no-global-shadow rationale
- §Sponge History Filtering: __fish_sponge_extra_sensitive name tokens (--entire
  match) and the tuning vars surfaced on the Sponge page
- config-settings function reference: four-page model, Tab/Shift-Tab, value rows
- index + README updated

docs/fish-config.index

@@ -24,6 +24,9 @@ toolhomes=## Tool Homes (XDG-compliant)
 editor=## Editor and Pager
 pager=## Editor and Pager
 scrollback=## Scrollback History
+__fish_scrollback_history_dir=## Scrollback History
+__fish_scrollback_history_max_files=## Scrollback History
+scrollback-dir=## Scrollback History
 
 # ── Section 2: PATH ───────────────────────────────────────────
 path=# 2. PATH SETUP
@@ -258,6 +261,9 @@ sponge=## Fisher-Managed Plugins
 sponge-filtering=## Sponge History Filtering
 history-filtering=## Sponge History Filtering
 privacy=## Sponge History Filtering
+__fish_sponge_extra_sensitive=## Sponge History Filtering
+extra-sensitive=## Sponge History Filtering
+sponge-settings=### config-settings
 bundled-plugins=## Bundled Plugin Functionality
 fish-plugins-manifest=## fish_plugins Manifest