Add notes about fixeddifficulty & ethash (#491)

Signed-off-by: Tim Beiko <[email protected]>

* fix trailing spaces
Signed-off-by: Nicolas MASSART <[email protected]>
Co-authored-by: Nicolas MASSART <[email protected]>

Author: timbeiko

docs/Reference/Config-Items.md

@@ -90,7 +90,7 @@ In private networks, the milestone block defines the protocol version for the ne
 
 ## Fixed difficulty
 
-Use `fixeddifficulty` to specify a fixed difficulty in private networks using Ethash.
+Use `fixeddifficulty` to specify a fixed difficulty in private networks using Ethash. This will keep the network's difficulty constant and override the `difficulty` parameter from the genesis file.
 
 !!! example
 
@@ -106,3 +106,6 @@ Use `fixeddifficulty` to specify a fixed difficulty in private networks using Et
       ...
     }
     ```
+
+!!! tip
+    Using `fixeddifficulty` is not recommended for use with Ethash outside of test environments. For production networks using Ethash, we recommend setting a low `difficulty` value in the genesis file instead. Ethash will adjust the difficulty of the network based on hashrate to produce blocks at the targetted frequency.