home: vim: document 'commentary' mappings

By using 'which-key'.

home/vim/after/plugin/mappings/commentary.vim

@@ -0,0 +1,12 @@
+lua << EOF
+local wk = require("which-key")
+
+local keys = {
+    name = "Comment/uncomment",
+    c = "Current line",
+    u = "Uncomment the current and adjacent commented lines",
+    ["gc"] = "Uncomment the current and adjacent commented lines",
+}
+
+wk.register(keys, { prefix = "gc" })
+EOF