Make code role highlighting consistent with code-block directive

Fixes https://github.com/sphinx-doc/sphinx/issues/5157

This is factored out of the sphinx-immaterial theme:
1ef121a612/sphinx_immaterial/inlinesyntaxhighlight.py (L1)

See also:
https://github.com/sphinx-doc/sphinx/pull/6916

doc/usage/restructuredtext/directives.rst

@@ -499,8 +499,10 @@ __ https://pygments.org/docs/lexers
    The directive's alias name :rst:dir:`sourcecode` works as well.  This
    directive takes a language name as an argument.  It can be `any lexer alias
    supported by Pygments <https://pygments.org/docs/lexers/>`_.  If it is not
-   given, the setting of :rst:dir:`highlight` directive will be used.
-   If not set, :confval:`highlight_language` will be used.
+   given, the setting of :rst:dir:`highlight` directive will be used.  If not
+   set, :confval:`highlight_language` will be used.  To display a code example
+   *inline* within other text, rather than as a separate block, you can use the
+   :rst:role:`code` role instead.
 
    .. versionchanged:: 2.0
       The ``language`` argument becomes optional.

doc/usage/restructuredtext/roles.rst

@@ -276,6 +276,34 @@ The following role creates a cross-reference to a term in a
    If you use a term that's not explained in a glossary, you'll get a warning
    during build.
 
+Inline code highlighting
+------------------------
+
+.. rst:role:: code
+
+   An *inline* code example.  When used directly, this role just displays the
+   text *without* syntax highlighting, as a literal.
+
+   .. code-block:: rst
+
+      By default, inline code such as :code:`1 + 2` just displays without
+      highlighting.
+
+   Unlike the :rst:dir:`code-block` directive, this role does not respect the
+   default language set by the :rst:dir:`highlight` directive.
+
+   To enable syntax highlighting, you must first use the ``role`` directive to
+   define a custom ``code`` role for a particular language:
+
+   .. code-block:: rst
+
+      .. role:: python(code)
+         :language: python
+
+      In Python, :python:`1 + 2` is equal to :python:`3`.
+
+   To display a multi-line code example, use the :rst:dir:`code-block` directive
+   instead.
 
 Math
 ----