Update libmpdec-related instructions

Author: hugovk

getting-started/setup-building.rst

@@ -841,8 +841,11 @@ some of CPython's modules (for example, ``zlib``).
                inetutils-inetd
 
       Note that Debian 12 and Ubuntu 24.04 do not have the ``libmpdec-dev``
-      package.  You can safely remove it from the install list above and the
-      Python build will use a bundled version.  But we recommend using the system
+      package.  You can remove it from the install list above: for Python 3.15
+      and older, the build will use a bundled version; for Python 3.16 and
+      newer, the bundled copy has been removed and the :mod:`decimal` module
+      will fall back to its slower pure-Python implementation.  We recommend
+      using the system
       `libmpdec <https://www.bytereef.org/mpdecimal/doc/libmpdec/>`__ library.
       Either build it from sources or install this package from
       https://deb.sury.org.
@@ -919,8 +922,7 @@ some of CPython's modules (for example, ``zlib``).
             $ GDBM_CFLAGS="-I$(dirname $(dirname $(which port)))/include" \
                GDBM_LIBS="-L$(dirname $(dirname $(which port)))/lib -lgdbm" \
                ./configure --config-cache \
-                           --with-pydebug \
-                           --with-system-libmpdec
+                           --with-pydebug
 
       .. tab:: Python 3.11-3.12
 

security/sbom.rst

@@ -10,8 +10,8 @@ that the CPython project uses is `SPDX <https://spdx.github.io/spdx-spec/v2.3/>`
 which can be transformed into other formats if necessary by consumers.
 
 There are multiple sources of third-party dependencies for CPython.
-Some are vendored into the source code of CPython itself (like ``mpdecimal``
-vendored at :cpy-file:`Modules/_decimal/libmpdec`) or they could be optionally pulled
+Some are vendored into the source code of CPython itself (like ``expat``
+vendored at :cpy-file:`Modules/expat`) or they could be optionally pulled
 in during builds like Windows using dependencies from the
 `python/cpython-source-deps <https://github.com/python/cpython-source-deps>`__
 repository.