docs(functions): split RETURNS into EXIT STATUS and stdout RETURNS

RETURNS previously conflated fish's $status exit code with genuine
stdout/printed output, e.g. rm listing "0/1" as if they were print
values rather than exit codes. Rename RETURNS to EXIT STATUS across
all 83 documented functions, and reintroduce RETURNS as a distinct
label reserved for the 15 functions that actually print to stdout.

Update build-manual.py's ENTRY_HEADS to render Exit Status before
Returns, manualtools.py's SECTIONS constant, and AGENTS.md's label
order and label-usage guidance to match. Add two verify-manual.py
regression tests: EXIT STATUS bodies must never contain stray
stdout/printed language, and Returns: must always render after
Exit Status: when both are present. Regenerate docs/fish-config.md.

functions/gi.fish

@@ -22,10 +22,14 @@
 #   -s, --stdout       Print API output to stdout instead of .gitignore
 #   targets            Comma- or space-separated list of language/tool names
 #
-# RETURNS
-#   0  Patterns appended or printed
+# EXIT STATUS
+#   0  Patterns appended, or resolved with -s/--stdout or -l/--list
 #   1  Not in a git repository or API fetch failed
 #
+# RETURNS
+#   With -s/--stdout, the fetched .gitignore pattern text, printed to stdout.
+#   With -l/--list, the supported target list, printed to stdout.
+#
 # EXAMPLE
 #   gi python,venv
 #   gi -b -p