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_real_command.fish

@@ -18,9 +18,12 @@
 # ARGUMENTS
 #   name  Command name to resolve (e.g. paru, yay)
 #
+# EXIT STATUS
+#   0  A non-wrapper binary was found
+#   1  No non-wrapper binary found on PATH
+#
 # RETURNS
-#   0  Real binary path printed to stdout
-#   1  No non-wrapper binary found on PATH (nothing printed)
+#   The resolved binary's real path, printed to stdout (nothing on failure)
 #
 # EXAMPLE
 #   set -l real (__fish_real_command paru)   # -> /usr/bin/paru, not the shim