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/__fish_variable_check.fish

@@ -12,7 +12,7 @@
 # ARGUMENTS
 #   variable_name  Name of the variable to check (without $ prefix)
 #
-# RETURNS
+# EXIT STATUS
 #   0  True (opt-in): value matches 1, true, yes, on, or y
 #   1  False (opt-out): value matches 0, false, no, off, or n
 #   2  Empty: variable is unset or empty