feat(edit): stop forcing VISUAL to the terminal editor

VISUAL was hardwired to $EDITOR (nvim), so 'edit --visual' would launch a
terminal editor detached with no tty. Comment it out so --visual falls
through to the GUI fallback chain, and let users set a real GUI editor via
local.fish. Update the editor-variable docs to match.

config.fish

@@ -108,7 +108,7 @@ if type -q nvim
 else
     set -gx EDITOR (command -s vi)
 end
-set -gx VISUAL $EDITOR
+# set -gx VISUAL $EDITOR # <- Use local.fish to set your preferred GUI editor.
 set -gx SUDO_EDITOR $EDITOR
 
 #   ──────────────────────────── GPG variables ─────────────────────────────