docs: document setting foldmethod alongside foldexpr (#8187)

doc/nvim-treesitter.txt

@@ -45,6 +45,7 @@ Treesitter features for installed languages need to be enabled manually in a
       vim.treesitter.start()
       -- folds, provided by Neovim
       vim.wo.foldexpr = 'v:lua.vim.treesitter.foldexpr()'
+      vim.wo.foldmethod = 'expr'
       -- indentation, provided by nvim-treesitter
       vim.bo.indentexpr = "v:lua.require'nvim-treesitter'.indentexpr()"
     end,