docs(functions): add man-page headers to branch, fc, gitup, sudo-toggle

These four were the only documented functions with no comment header,
carrying just a one-line description. Content is merged from their manual
entries plus what the implementations actually do (fc's vi fallback and
empty-buffer abort, sudo-toggle's credential-cache clear), so the headers
are a superset of Section 5 rather than a copy of it.

functions/gitup.fish

@@ -1,4 +1,26 @@
-# Fetch updates and show git status
+# Copyright (C) 2026 Rootiest
+# SPDX-License-Identifier: AGPL-3.0-or-later
+
+# CATEGORY
+#   04-git-and-version-control
+#
+# SYNOPSIS
+#   gitup [args...]
+#
+# DESCRIPTION
+#   Fetches updates from the remote and shows git status. Extra arguments
+#   are forwarded to git fetch.
+#
+# ARGUMENTS
+#   args...   Forwarded verbatim to git fetch
+#
+# RETURNS
+#   0  Fetch and status succeeded
+#   1  Not inside a git work tree
+#
+# EXAMPLE
+#   gitup
+#   gitup --all
 function gitup --description 'Fetch updates and show git status'
     # Check if we are even in a git repository
     if not git rev-parse --is-inside-work-tree >/dev/null 2>&1