Update RC spellcheck installation information

Update Roundcube spellcheck installation information, modern browsers include built in spell check, so this maybe a unwanted feature for some.
Also bullet pointed the important information about `config.inc.php`

docs/third_party-roundcube.md

@@ -9,7 +9,7 @@ mv roundcubemail-1.5-rc rc
 chown -R root: rc/
 ```
 
-If you need spell check features, create a file `data/hooks/phpfpm/aspell.sh` with the following content, then `chmod +x data/hooks/phpfpm/aspell.sh`. This installs a local spell check engine.
+If you need spell check features, create a file `data/hooks/phpfpm/aspell.sh` with the following content, then `chmod +x data/hooks/phpfpm/aspell.sh`. This installs a local spell check engine. Note, most modern web browsers have built in spell check, so you may not want/need this.
 ```
 #!/bin/bash
 apk update
@@ -17,9 +17,9 @@ apk add aspell-en # or any other language
 ```
 
 Create a file `data/web/rc/config/config.inc.php` with the following content.
-
-**Change the `des_key` parameter to a random value.** It is used to temporarily store your IMAP password. The "db_prefix" is optional but recommended.
-
+   - **Change the `des_key` parameter to a random value.** It is used to temporarily store your IMAP password.
+   - The `db_prefix` is optional but recommended.
+   - If you didn't install spell check in the above step, remove `spellcheck_engine` parameter and replace it with `$config['enable_spellcheck'] = false;`.
 ```
 <?php
 error_reporting(0);