docs: document the two-source SSOT split

Function documentation now comes from functions/*.fish comment headers;
everything else stays in docs/manual/**. Records that split in the three
places a contributor looks:

- AGENTS.md "Documentation Policy" and Coding Convention #4, which now
  spells out the full label set and flags CATEGORY as required-to-publish.
- Manual section 11, which pointed readers at docs/manual/ for everything.
- README, same.

docs/fish-config.md

@@ -3054,3 +3054,14 @@ editor, or from a shell:
 
     cd ~/.config/fish/docs/manual
     grep -rn "keybindings" .
+
+Section 5 is the exception. Function entries are generated from the
+man-page-style comment header above each function in `functions/*.fish`,
+so the documentation for a command lives beside the code that implements
+it and cannot drift from it. To read the source for a single function, or
+to correct its documentation, open the function itself:
+
+    functions/git-clean.fish
+
+The files under `docs/manual/05-functions/` carry only the category
+titles, ordering, and search keywords.

docs/manual/11-viewing-this-manual.md

@@ -83,3 +83,14 @@ editor, or from a shell:
 
     cd ~/.config/fish/docs/manual
     grep -rn "keybindings" .
+
+Section 5 is the exception. Function entries are generated from the
+man-page-style comment header above each function in `functions/*.fish`,
+so the documentation for a command lives beside the code that implements
+it and cannot drift from it. To read the source for a single function, or
+to correct its documentation, open the function itself:
+
+    functions/git-clean.fish
+
+The files under `docs/manual/05-functions/` carry only the category
+titles, ordering, and search keywords.