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_read_key.fish

@@ -21,15 +21,18 @@
 # ARGUMENTS
 #   (none)
 #
-# RETURNS
-#   0  A key was read; one of these tokens is printed to stdout:
-#        up down left right   arrow keys
-#        space tab backtab enter escape backspace
-#        quit                 Ctrl-C (byte 3) in raw mode
-#        <char>               any other single printable character
-#        ""                   nothing decodable was read
+# EXIT STATUS
+#   0  A key was read
 #   1  The terminal could not be put into raw mode (stdin is not a TTY)
 #
+# RETURNS
+#   One token, printed to stdout:
+#     up down left right   arrow keys
+#     space tab backtab enter escape backspace
+#     quit                 Ctrl-C (byte 3) in raw mode
+#     <char>               any other single printable character
+#     ""                   nothing decodable was read
+#
 # EXAMPLE
 #   set -l key (__config_settings_read_key)
 #   or return                       # not a TTY — bail