Rewrite `usage/configuration.rst` (#12537)

- Remove now-removed configuration values
- Use obvious 'unset' defaults for ``project`` and ``author``
- Prefer 'e.g.' to 'ex.'
- Allow non-list sequence types in various configuration values
- Add types and defaults to every confval directive

doc/usage/restructuredtext/directives.rst

@@ -49,8 +49,8 @@ tables of contents.  The ``toctree`` directive is the central element.
    indicate the depth of the tree; by default, all levels are included. [#]_
 
    The representation of "TOC tree" is changed in each output format.  The
-   builders that output multiple files (ex. HTML) treat it as a collection of
-   hyperlinks.  On the other hand, the builders that output a single file (ex.
+   builders that output multiple files (e.g. HTML) treat it as a collection of
+   hyperlinks.  On the other hand, the builders that output a single file (e.g.
    LaTeX, man page, etc.) replace it with the content of the documents on the
    TOC tree.
 
@@ -1060,10 +1060,11 @@ Including content based on tags
 
       .. only:: html and draft
 
-   Undefined tags are false, defined tags (via the ``-t`` command-line option or
-   within :file:`conf.py`, see :ref:`here <conf-tags>`) are true.  Boolean
-   expressions, also using parentheses (like ``(latex or html) and draft``) are
-   supported.
+   Undefined tags are false, defined tags are true
+   (tags can be defined via the :option:`--tag <sphinx-build --tag>`
+   command-line option or within :file:`conf.py`, see :ref:`here <conf-tags>`).
+   Boolean expressions (like ``(latex or html) and draft``) are supported
+   and may use parentheses.
 
    The *format* and the *name* of the current builder (``html``, ``latex`` or
    ``text``) are always set as a tag [#]_.  To make the distinction between