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/gip6.fish

@@ -11,10 +11,13 @@
 #   Fetches and prints the machine's public IPv6 address using icanhazip.com.
 #   Prints an error message if IPv6 is unavailable on the current network.
 #
-# RETURNS
-#   0  IPv6 address printed
+# EXIT STATUS
+#   0  IPv6 address resolved
 #   1  IPv6 unavailable or not supported on this network
 #
+# RETURNS
+#   The machine's public IPv6 address, printed to stdout
+#
 # EXAMPLE
 #   gip6
 function gip6 --description 'Get public IPv6 address'