#21720: Update "directory server options" preamble in manpage

changes/bug21720

@@ -0,0 +1,5 @@
+  o Documentation:
+    - Update the description of the directory server options in the manual
+      page, to clarify that DirPort is no longer necessary to be a directory
+      cache.  Closes ticket 21720.
+

doc/tor.1.txt

@@ -2121,8 +2121,9 @@ is non-zero):
 DIRECTORY SERVER OPTIONS
 ------------------------
 
-The following options are useful only for directory servers (that is,
-if DirPort is non-zero):
+The following options are useful only for directory servers. (Relays with
+enough bandwidth automatically become directory servers; see DirCache for
+details.)
 
 [[DirPortFrontPage]] **DirPortFrontPage** __FILENAME__::
     When this option is set, it takes an HTML file and publishes it as "/" on