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/__config_settings_get_raw.fish

@@ -12,8 +12,11 @@
 # ARGUMENTS
 #   varname  Variable name without the $ prefix
 #
+# EXIT STATUS
+#   0  Always
+#
 # RETURNS
-#   0  Always; prints the value or "DEFAULT" to stdout
+#   The value, or "DEFAULT" if unset, printed to stdout
 #
 # EXAMPLE
 #   set v (__config_settings_get_raw sponge_delay)   # "2" or "DEFAULT"