Restore documentation for approved-routers

We removed this documentation in 607724c696a6e, when we removed
Naming Authoritative Directories, but actually this file is still
used by authorities to indicate rejected and invalid fingerprints.

Closes ticket 21148.

changes/21148

@@ -0,0 +1,4 @@
+  o Documentation:
+    - Restore documentation for the authorities' "approved-routers" file.
+      Closes ticket 21148.
+

doc/tor.1.txt

@@ -2868,6 +2868,16 @@ __DataDirectory__**/hashed-fingerprint**::
     Only used by bridges. Holds the hashed fingerprint of the bridge's
     identity key. (That is, the hash of the hash of the identity key.)
 
+__DataDirectory__**/approved-routers**::
+    Only used by authoritative directory servers. This file lists
+    the status of routers by their identity fingerprint.
+    Each line lists a status and a fingerprint separated by
+    whitespace. See your **fingerprint** file in the __DataDirectory__ for an
+    example line. If the status is **!reject** then descriptors from the
+    given identity (fingerprint) are rejected by this server. If it is
+    **!invalid** then descriptors are accepted but marked in the directory as
+    not valid, that is, not recommended.
+
 __DataDirectory__**/v3-status-votes**::
     Only for v3 authoritative directory servers. This file contains
     status votes from all the authoritative directory servers.