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/branch.fish

@@ -1,4 +1,25 @@
-# Switch to or create a git branch
+# Copyright (C) 2026 Rootiest
+# SPDX-License-Identifier: AGPL-3.0-or-later
+
+# CATEGORY
+#   04-git-and-version-control
+#
+# SYNOPSIS
+#   branch <branch_name>
+#
+# DESCRIPTION
+#   Switches to a local git branch, creating it if it does not already
+#   exist. Extra arguments are forwarded to git checkout.
+#
+# ARGUMENTS
+#   branch_name   Branch to switch to or create
+#
+# RETURNS
+#   0  Branch checked out or created
+#   1  Not inside a git work tree
+#
+# EXAMPLE
+#   branch feature/new-ui
 function branch --description 'Switch to or create a git branch'
     if not git rev-parse --is-inside-work-tree >/dev/null 2>&1
         echo "Not a git repo."