Added documentation about 413 errors with an nginx solution (#15313)

* Added documentation about 413 errors with an nginx solution.

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Norwin <noerw@users.noreply.github.com>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>

docs/content/doc/help/faq.en-us.md

@@ -85,6 +85,12 @@ If certain clone options aren't showing up (HTTP/S or SSH), the following option
 `DISABLE_SSH`: if set to true, there will be no SSH link  
 `SSH_EXPOSE_ANONYMOUS`: if set to false, SSH links will be hidden for anonymous users
 
+## File upload fails with: 413 Request Entity Too Large
+
+This error occurs when the reverse proxy limits the file upload size.
+
+See the [reverse proxy guide]({{< relref "doc/usage/reverse-proxies.en-us.md" >}}) for a solution with nginx.
+
 ## Custom Templates not loading or working incorrectly
 
 Gitea's custom templates must be added to the correct location or Gitea will not find and use them.  

docs/content/doc/usage/reverse-proxies.en-us.md

@@ -120,6 +120,14 @@ server {
 }
 ```
 
+## Resolving Error: 413 Request Entity Too Large
+
+This error indicates nginx is configured to restrict the file upload size.
+
+In your nginx config file containing your Gitea proxy directive, find the `location { ... }` block for Gitea and add the line
+`client_max_body_size 16M;` to set this limit to 16 megabytes or any other number of choice.
+
+
 ## Apache HTTPD
 
 If you want Apache HTTPD to serve your Gitea instance, you can add the following to your Apache HTTPD configuration (usually located at `/etc/apache2/httpd.conf` in Ubuntu):