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/__kitty_logging_dir.fish

@@ -9,8 +9,11 @@
 #   order: $KITTY_CONFIG_DIRECTORY, else $XDG_CONFIG_HOME/kitty, else
 #   ~/.config/kitty.
 #
+# EXIT STATUS
+#   0  Always
+#
 # RETURNS
-#   0  Always (path printed to stdout)
+#   The Kitty configuration directory path, printed to stdout
 #
 # EXAMPLE
 #   set -l dir (__kitty_logging_dir)