docs(functions): standardize all function doc blocks to UNIX man-page style

Replace all ad-hoc inline comments between license headers and function
declarations with consistent SYNOPSIS / DESCRIPTION / ARGUMENTS / RETURNS /
EXAMPLE blocks across all 99 project-owned functions/ files. No executable
logic, variable names, or exit codes were modified.

Completes Task #6 from AGENTS.md (Retroactive Function Documentation
Standardization).

functions/fzf-update.fish

@@ -1,7 +1,15 @@
 # Copyright (C) 2026 Rootiest
 # SPDX-License-Identifier: AGPL-3.0-or-later
 
-# Installs or upgrades fzf from git HEAD into ~/.fzf
+# SYNOPSIS
+#   fzf-update
+#
+# DESCRIPTION
+#   Installs or upgrades fzf from git HEAD into ~/.fzf. Pulls the latest
+#   changes if ~/.fzf already exists, or clones the repository if not.
+#
+# EXAMPLE
+#   fzf-update
 function fzf-update --description 'Install or upgrade fzf from git HEAD'
     if test -d ~/.fzf
         echo "Updating fzf..."