fix: incorrect blockchain.relayfee description

The description of `blockchain.relayfee` promises the user that
transactions paying the relayfee will get accepted into the servers
mempool. This however seems incorrect as the relayfee is just a
fixed value in the core backend (either configured or a hardcoded default value)
and doesn't change with a growing mempool.
See definition:
https://github.com/laanwj/bitcoin/blob/d387b8ec15e553db9b9c370314ee359e981fb374/src/main.cpp#L55-L56
and usage for the API:
https://github.com/laanwj/bitcoin/blob/d387b8ec15e553db9b9c370314ee359e981fb374/src/rpcmisc.cpp#L86-L87

To get the minimal feerate to be paid for mempool acceptance one has to
use getmempoolinfo['mempoolminfee'].

Author: f321x

docs/protocol-methods.rst

@@ -237,8 +237,9 @@ Subscribe to receive block headers when a new block is found.
 blockchain.relayfee
 ===================
 
-Return the minimum fee a low-priority transaction must pay in order to
-be accepted to the daemon's memory pool.
+Feerates lower than this are considered zero fee and are not being
+relayed to the bitcoin network by the server.
+This feerate does not guarantee acceptance into the mempool of the server.
 
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