home: vim: document 'treesitter-textobjects' maps

By using 'which-key'.

home/vim/after/plugin/mappings/tree-sitter-textobjects.vim

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+lua << EOF
+local wk = require("which-key")
+
+local keys = {
+    ["aa"] = "a parameter",
+    ["ia"] = "inner parameter",
+    ["ab"] = "a block",
+    ["ib"] = "inner block",
+    ["ac"] = "a class",
+    ["ic"] = "inner class",
+    ["af"] = "a function",
+    ["if"] = "inner function",
+    ["ak"] = "a comment",
+}
+
+wk.register(keys, { mode = "o" })
+EOF