Document --fg-aircraft in the command-line help.

Translations/strings-default.xml

@@ -75,8 +75,9 @@
  <verbose-desc>Show all command line options when combined with --help or -h</verbose-desc>
  <version-desc>Display the current FlightGear version</version-desc>
  <fg-root-desc>Specify the root data path</fg-root-desc>
- <fg-scenery-desc n="0">Specify the base scenery path;</fg-scenery-desc>
+ <fg-scenery-desc n="0">Specify the scenery path(s);</fg-scenery-desc>
  <fg-scenery-desc n="1">Defaults to $FG_ROOT/Scenery</fg-scenery-desc>
+ <fg-aircraft-desc>Specify additional aircraft directory path(s)</fg-aircraft-desc>
  <language-desc>Select the language for this session</language-desc>
  <disable-game-mode-desc>Disable full-screen game mode</disable-game-mode-desc>
  <enable-game-mode-desc>Enable full-screen game mode</enable-game-mode-desc>

options.xml

@@ -77,6 +77,13 @@
     <brief/>
    </option>
 
+   <option>
+       <name>fg-aircraft</name>
+       <arg>path</arg>
+       <description>strings/fg-aircraft-desc</description>
+       <brief/>
+      </option>
+
    <option>
     <name>language</name>
     <arg>code</arg>
@@ -537,11 +544,12 @@
     <description>strings/disable-auto-coordination-desc</description>
    </option>
 
+<!--
    <option>
     <name>aircraft-dir</name>
     <arg>path</arg>
     <description>strings/aircraft-dir-desc</description>
-   </option>
+   </option> -->
 
    <option>
      <name>livery</name>