Clarify that pyproject.toml config will not be removed

The current documentation regarding `pyproject.toml` claims that
support for using it to specify package metadata and build
configuration options might be "completely removed" in a future
release.

However, nowadays it is clear
(pypa#1688 (comment)
and pypa#3214) that `pyproject.toml` is not going anywhere, is here
to stay, and is in fact expected to be the preferred format
in the future.

This makes an incremental change toward that future by clarifying
that `pyproject.toml` support will not be removed in a future
release of setuptools.

docs/userguide/dependency_management.rst

@@ -439,5 +439,5 @@ This can be configured as shown in the example below.
    While the ``[build-system]`` table should always be specified in the
    ``pyproject.toml`` file, support for adding package metadata and build configuration
    options via the ``[project]`` and ``[tool.setuptools]`` tables is still
-   experimental and might change (or be completely removed) in future releases.
+   experimental and might change in future releases.
    See :doc:`/userguide/pyproject_config`.

docs/userguide/package_discovery.rst

@@ -567,8 +567,8 @@ The project layout remains the same and ``setup.cfg`` remains the same.
 
 .. [#experimental]
    Support for specifying package metadata and build configuration options via
-   ``pyproject.toml`` is experimental and might change (or be completely
-   removed) in the future. See :doc:`/userguide/pyproject_config`.
+   ``pyproject.toml`` is experimental and might change
+   in the future. See :doc:`/userguide/pyproject_config`.
 .. [#layout1] https://blog.ionelmc.ro/2014/05/25/python-packaging/#the-structure
 .. [#layout2] https://blog.ionelmc.ro/2017/09/25/rehashing-the-src-layout/
 

docs/userguide/pyproject_config.rst

@@ -10,7 +10,7 @@ Configuring setuptools using ``pyproject.toml`` files
    Support for declaring :doc:`project metadata
    <PyPUG:specifications/declaring-project-metadata>` or configuring
    ``setuptools`` via ``pyproject.toml`` files is still experimental and might
-   change (or be removed) in future releases.
+   change in future releases.
 
 .. important::
    For the time being, ``pip`` still might require a ``setup.py`` file

docs/userguide/quickstart.rst

@@ -410,5 +410,5 @@ up-to-date references that can help you when it is time to distribute your work.
    While the ``[build-system]`` table should always be specified in the
    ``pyproject.toml`` file, support for adding package metadata and build configuration
    options via the ``[project]`` and ``[tool.setuptools]`` tables is still
-   experimental and might change (or be completely removed) in future releases.
+   experimental and might change in future releases.
    See :doc:`/userguide/pyproject_config`.