docs: describe the new warning levels

When -Wextra was added, the warning levels changed their meaning. Add a
description in the build option section and leave the security hardening
section as mostly a pointer to it.

Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
Change-Id: Iabf2f598d0bf3e865c9b991c5d44d2acb9572bd5

docs/process/security-hardening.rst

@@ -131,38 +131,9 @@ Several build options can be used to check for security issues. Refer to the
   overflows.
 
 - The ``W`` build flag can be used to enable a number of compiler warning
-  options to detect potentially incorrect code.
-
-  - W=0 (default value)
-
-    The ``Wunused`` with ``Wno-unused-parameter``, ``Wdisabled-optimization``
-    and ``Wvla`` flags are enabled.
-
-    The ``Wunused-but-set-variable``, ``Wmaybe-uninitialized`` and
-    ``Wpacked-bitfield-compat`` are GCC specific flags that are also enabled.
-
-  - W=1
-
-    Adds ``Wextra``, ``Wmissing-format-attribute``, ``Wmissing-prototypes``,
-    ``Wold-style-definition`` and ``Wunused-const-variable``.
-
-  - W=2
-
-    Adds ``Waggregate-return``, ``Wcast-align``, ``Wnested-externs``,
-    ``Wshadow``, ``Wlogical-op``.
-
-  - W=3
-
-    Adds ``Wbad-function-cast``, ``Wcast-qual``, ``Wconversion``, ``Wpacked``,
-    ``Wpointer-arith``, ``Wredundant-decls`` and
-    ``Wswitch-default``.
-
-  Refer to the GCC or Clang documentation for more information on the individual
-  options: https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html and
-  https://clang.llvm.org/docs/DiagnosticsReference.html.
-
-  NB: The ``Werror`` flag is enabled by default in TF-A and can be disabled by
-  setting the ``E`` build flag to 0.
+  options to detect potentially incorrect code. TF-A is tested with ``W=0`` but
+  it is recommended to develop against ``W=2`` (which will eventually become the
+  default).
 
 .. rubric:: References