doc: add info about timeoutlen (nvim-lua#691)

Add separate comment for `timeoutlen` option
`timeoutlen` option was under unrelated comment with `updatetime` option.

Author: melobern

init.lua

@@ -130,6 +130,9 @@ vim.opt.signcolumn = 'yes'
 
 -- Decrease update time
 vim.opt.updatetime = 250
+
+-- Decrease mapped sequence wait time
+-- Displays which-key popup sooner
 vim.opt.timeoutlen = 300
 
 -- Configure how new splits should be opened