Adding documentation for the logout command

docs/using-concourse/fly-cli.any

@@ -78,6 +78,28 @@ flag. Once you've learned what the commands do, you may want to consult
   \code{example} Concourse instance.
 }
 
+\section{\code{logout}\aux{: Remove authentication and delete Concourse targets}}{fly-logout}{
+  There are cases when you would like to remove all evidence of a particular
+  target. This is achieved by the \code{logout} command. There are two variants
+  of this command, one to get rid of a specific target, and another to remove
+  all targets from the \code{~/.flyrc} file.
+
+  To remove a specific target run:
+
+  \codeblock{bash}{
+  $ fly -t example logout
+  }
+
+  To remove all targets run:
+
+  \codeblock{bash}{
+  $ fly logout -a
+  }
+
+  Note: These two variations are mutually exclusive. If the target parameter
+  \code{-t} and all parameter \code{-a} are both specified, an error will occur.
+}
+
 \section{\code{targets}\aux{: List the current targets}}{fly-targets}{
   To see what targets are currently known to \code{fly}, run: