Adding docs on troubleshooting Live Preview http auth issues

docs/control-panel/create.md

@@ -192,10 +192,12 @@ Saves the entry and returns the [Entry Manager](control-panel/entry-manager.md)
 
 ### Preview
 
-A live preview of the entry is available if the `channel_prefs_preview_url` is set **or** the [Pages Module](add-ons/pages.md) is installed and a Page URI and template have been set.
+A live preview of the entry is available if the relavant Channel's [Preview URL](control-panel/channels.md#settings-tab) is set **or** the [Pages Module](add-ons/pages.md) is installed and a Page URI and template have been set.
 
 If neither is set, the preview button will have an exclamation mark (!) and will link to channel preferences page where Preview URL can be set.
 
 The preview will open a split screen that allows a live preview of edits. The template used to display the preview is based on the Page fields if set and the channel preview URL otherwise.
 
 When the preview is triggered, it is being displayed side-by-side with edit screen. The size of preview container can be adjusted with mouse dragging its border. The preview is dynamically being updated as you change the fields.
+
+Live Preview does require the [HTTP Authorization Header be enabled](troubleshooting/error-messages.md#http-authorization-header-missing).

docs/installation/requirements.md

@@ -117,6 +117,12 @@ If you are hosted on an Apache server, the `AcceptPathInfo` option needs to be e
 - Ask your web host or server admin to enable the option
 - Set your site's URLs to use [query strings](general/url-structure.md#query-strings)
 
+If on an Apache server and [Live Preview](control-panel/create.md#preview) shows an error message 'HTTP Authorization Header Missing', you will need to add this line of code to your your [.htaccess file](installation/best-practices.md#1-create-an-htaccess-file)
+
+```php
+SetEnvIf Authorization "(.*)" HTTP_AUTHORIZATION=$1
+```
+
 ### URL Segment Support
 
 If the [Server Compatibility Wizard](#server-compatibility-wizard) lists URL Segment Support as _Unsupported_, you will need to set your site's URLs to use [query strings](general/url-structure.md#query-strings).

docs/troubleshooting/error-messages.md

@@ -45,6 +45,17 @@ This error---and similar warnings such as **"Warning: imagettftext(): Could not
 
 If that is not an option, **Use TrueType Font for Captcha?** should be set to _No_ under `Settings --> CAPTCHA` or CAPTCHAs should be disabled altogether.
 
+## HTTP Authorization Header Missing
+
+ExpressionEngine returns the following error message when attempting to use [Live Preview](control-panel/create.md#preview): **"HTTP Authorization Header Missing"**
+
+### Troubleshooting
+
+The Authorization header is stripped by some Apache servers, causing an error rendering the Live Preview. This behavior may be overriden in your [.htaccess file](installation/best-practices.md#1-create-an-htaccess-file) by adding the following line:
+
+```php
+SetEnvIf Authorization "(.*)" HTTP_AUTHORIZATION=$1
+```
 ## Warning: Cannot modify header information
 
 ExpressionEngine returns the following warning: **"Cannot modify header information."**