doc: document that urgent doesn't use the 2-block estimate, but the 6…

…-block.

Turns out this was accidentally changed for v0.10.1 in
d8e6889 where we made fee levels less
aggressive.

Oops.  I guess we can fix the docs.  And we now have "2blocks" if you
want it really fast!

Closes: #6129
Changelog-Fixed: JSON-RPC: `feerates` document correctly that urgent means 6 blocks (not 2), and give better feerate examples.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

doc/lightning-feerates.7.md

@@ -104,8 +104,8 @@ NOTES
 Many other commands have a *feerate* parameter.  This can be:
 
 * One of the strings to use lightningd's internal estimates:
-  * *urgent* (aim for next block), 
-  * *normal* (next 6 blocks or so)
+  * *urgent* (next 6 blocks or so)
+  * *normal* (next 12 blocks or so)
   * *slow* (next 100 blocks or so)
   * *minimum* for the lowest value bitcoind will currently accept (added in v23.05)