Add <C-b>/<C-f> cmp mapping to scroll cmp docs (nvim-lua#750)

Author: RafaelZasas

init.lua

@@ -677,6 +677,10 @@ require('lazy').setup({
           -- Select the [p]revious item
           ['<C-p>'] = cmp.mapping.select_prev_item(),
 
+          -- scroll the documentation window [b]ack / [f]orward
+          ['<C-b>'] = cmp.mapping.scroll_docs(-4),
+          ['<C-f>'] = cmp.mapping.scroll_docs(4),
+
           -- Accept ([y]es) the completion.
           --  This will auto-import if your LSP supports it.
           --  This will expand snippets if the LSP sent a snippet.