[docs] Include setup instructions for environment variable KBN_PATH_CONF (#90794)

docs/setup/install/deb.asciidoc

@@ -156,6 +156,7 @@ locations for a Debian-based system:
 | config
   | Configuration files including `kibana.yml`
   | /etc/kibana
+  | <<settings,KBN_PATH_CONF>>
  d|
 
 | data

docs/setup/install/rpm.asciidoc

@@ -149,6 +149,7 @@ locations for an RPM-based system:
 | config
   | Configuration files including `kibana.yml`
   | /etc/kibana
+  | <<settings,KBN_PATH_CONF>>
  d|
 
 | data

docs/setup/install/targz.asciidoc

@@ -134,6 +134,7 @@ important data later on.
 | config
   | Configuration files including `kibana.yml`
   | $KIBANA_HOME\config
+  | <<settings,KBN_PATH_CONF>>
  d|
 
 | data

docs/setup/install/windows.asciidoc

@@ -81,6 +81,7 @@ important data later on.
 | config
   | Configuration files including `kibana.yml`
   | $KIBANA_HOME\config
+  | <<settings,KBN_PATH_CONF>>
  d|
 
 | data

docs/setup/settings.asciidoc

@@ -5,7 +5,12 @@ The {kib} server reads properties from the `kibana.yml` file on startup. The
 location of this file differs depending on how you installed {kib}. For example,
 if you installed {kib} from an archive distribution (`.tar.gz` or `.zip`), by
 default it is in `$KIBANA_HOME/config`. By default, with package distributions
-(Debian or RPM), it is in `/etc/kibana`.
+(Debian or RPM), it is in `/etc/kibana`.  The config directory can be changed via the
+`KBN_PATH_CONF` environment variable:
+
+```
+KBN_PATH_CONF=/home/kibana/config ./bin/kibana
+```
 
 The default host and port settings configure {kib} to run on `localhost:5601`. To change this behavior and allow remote users to connect, you'll need to update your `kibana.yml` file. You can also enable SSL and set a
 variety of other options. Finally, environment variables can be injected into