docs: add an entry about the setup-hook

doc/lspconfig.txt

@@ -266,6 +266,23 @@ The global defaults for all servers can be overridden by extending the
 <
 `setup {}` can additionally override these defaults in subsequent calls.
 
+==============================================================================
+SETUP HOOK                                                *lspconfig-setup-hook*
+
+`lspconfig` will execute the `on_setup` hook for each setup call to a server after
+validating its configuration, and before attempting to launch the server
+itself. One typical usage is to allow ad-hoc substitution for any
+configuration entry, such as `cmd`.
+
+>
+  local lspconfig = require 'lspconfig'
+  lspconfig.util.on_setup = util.add_hook_before(lspconfig.util.on_setup, function(config)
+    if some_condition then
+      local custom_server_prefix = "/my/custom/path"
+      config.cmd_env = custom_server_prefix
+    end
+  end)
+<
 ==============================================================================
 SERVER CONFIGURATIONS                                 *lspconfig-configurations*