docs: improve buffer completion from all open buffers

docs/recipes.md

@@ -39,18 +39,25 @@ completion = {
 
 ### Buffer completion from all open buffers
 
+The default behavior is to only show completions from **visible** "normal" buffers (i.e. it woudldn't include neo-tree). This will instead show completions from all buffers, even if they're not visible on screen. Note that the performance impact of this has not been tested. 
+
 ```lua
-        providers = {
-          buffer = {
-            name = "Buffer",
-            module = "blink.cmp.sources.buffer",
-            opts = {
-              get_bufnrs = function()
-                return vim.api.nvim_list_bufs()
-              end,
-            },
-          },
-        },
+sources = {
+  providers = {
+    buffer = {
+      opts = {
+        -- get all buffers, even ones like neo-tree
+        get_bufnrs = vim.api.nvim_list_bufs
+        -- or (recommended) filter to only "normal" buffers
+        get_bufnrs = function()
+          return vim.tbl_filter(function(bufnr)
+            return vim.bo[bufnr].buftype == ''
+          end, vim.api.nvim_list_bufs())
+        end
+      }
+    }
+  }
+}
 ```
 
 ### Don't show completion menu automatically in cmdline mode