Set default EVM version to Berlin

docs/using-the-compiler.rst

@@ -158,9 +158,12 @@ at each version. Backward compatibility is not guaranteed between each version.
    - Shifting operators use shifting opcodes and thus need less gas.
 - ``petersburg``
    - The compiler behaves the same way as with constantinople.
-- ``istanbul`` (**default**)
+- ``istanbul``
    - Opcodes ``chainid`` and ``selfbalance`` are available in assembly.
-- ``berlin`` (**experimental**)
+- ``berlin`` (**default**)
+   - Gas costs for ``SLOAD``, ``*CALL``, ``BALANCE``, ``EXT*`` and ``SELFDESTRUCT`` increased. The
+     compiler assumes cold gas costs for such operations. This is relevant for gas estimation and
+     the optimizer.
 
 
 .. _compiler-api: