Document Web3signer default heap (ethstaker#2560)

Author: yorickdowne

default.env

@@ -228,7 +228,7 @@ PROXY_EXTRAS=
 # to 12g. If left empty, the defaults in besu.yml and teku.yml are used
 BESU_HEAP=
 TEKU_HEAP=
-# Heap for Web3signer. Defaults to -Xmx4g; -Xmx2g should also work in many setups
+# Heap for Web3signer. Defaults to -Xmx6g.
 W3S_HEAP=
 # Heap for Lodestar. Sets NODE_OPTIONS to this value, for example --max-old-space-size=16384
 # If left empty, the default in lodestar.yml will be used