update PerConnBWRate/Burst manpage entries to not mention consensus

context: https://lists.torproject.org/pipermail/tor-relays/2018-February/014585.html

The manpage says that relays will use the value in consensus but the consensus does not longer include these values.

This patch removes "You should never need to change this value, since a network-wide value is published in the consensus and your relay will use that value."

and is the minimal change, but it would be better to add more information that says if tor has some built-in mechanism to avoid that a single client uses all bw. Also: How does tor tell non-relays from relays apart?

@@ -243,13 +243,11 @@
 
 [[PerConnBWRate]] **PerConnBWRate** __N__ **bytes**|**KBytes**|**MBytes**|**GBytes**|**TBytes**|**KBits**|**MBits**|**GBits**|**TBits**::
     If set, do separate rate limiting for each connection from a non-relay.
-    You should never need to change this value, since a network-wide value is
-    published in the consensus and your relay will use that value. (Default: 0)
+    (Default: 0)
 
 [[PerConnBWBurst]] **PerConnBWBurst** __N__ **bytes**|**KBytes**|**MBytes**|**GBytes**|**TBytes**|**KBits**|**MBits**|**GBits**|**TBits**::
     If set, do separate rate limiting for each connection from a non-relay.
-    You should never need to change this value, since a network-wide value is
-    published in the consensus and your relay will use that value. (Default: 0)
+    (Default: 0)
 
 [[ClientTransportPlugin]] **ClientTransportPlugin** __transport__ socks4|socks5 __IP__:__PORT__::
 **ClientTransportPlugin** __transport__ exec __path-to-binary__ [options]::