docs: Mention that low-level calls do not have an extcodesize check

docs/control-structures.rst

@@ -109,6 +109,8 @@ Due to the fact that the EVM considers a call to a non-existing contract to
 always succeed, Solidity uses the ``extcodesize`` opcode to check that
 the contract that is about to be called actually exists (it contains code)
 and causes an exception if it does not.
+Note that this check is not performed in case of :ref:`low-level calls <address_related>` which
+operate on addresses rather than contract instances.
 
 Function calls also cause exceptions if the called contract itself
 throws an exception or goes out of gas.

docs/units-and-global-variables.rst

@@ -262,6 +262,16 @@ For more information, see the section on :ref:`address`.
     to make safe Ether transfers, always check the return value of ``send``, use ``transfer`` or even better:
     Use a pattern where the recipient withdraws the money.
 
+.. warning::
+    Due to the fact that the EVM considers a call to a non-existing contract to always succeed,
+    Solidity includes an extra check using the ``extcodesize`` opcode when performing external calls.
+    This ensures that the contract that is about to be called either actually exists (it contains code)
+    or an exception is raised.
+
+    The low-level calls which operate on addresses rather than contract instances (i.e. ``.call()``,
+    ``.delegatecall()``, ``.staticcall()``, ``.send()`` and ``.transfer()``) **do not** include this
+    check, which makes them cheaper in terms of gas but also less safe.
+
 .. note::
    Prior to version 0.5.0, Solidity allowed address members to be accessed by a contract instance, for example ``this.balance``.
    This is now forbidden and an explicit conversion to address must be done: ``address(this).balance``.