docs: Document cross-referencing using relative path

Update the Cross-referencing section to explain how to create a
cross-reference to a document using relative paths and with no
additional syntax, by relying on automarkup.py.

Signed-off-by: Nícolas F. R. A. Prado <nfraprado@protonmail.com>
Link: https://lore.kernel.org/r/20210128010028.58541-3-nfraprado@protonmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/doc-guide/sphinx.rst

@@ -340,16 +340,26 @@ Rendered as:
 Cross-referencing
 -----------------
 
-Cross-referencing from one documentation page to another can be done by passing
-the path to the file starting from the Documentation folder.
-For example, to cross-reference to this page (the .rst extension is optional)::
+Cross-referencing from one documentation page to another can be done simply by
+writing the path to the document file, no special syntax required. The path can
+be either absolute or relative. For absolute paths, start it with
+"Documentation/". For example, to cross-reference to this page, all the
+following are valid options, depending on the current document's directory (note
+that the ``.rst`` extension is required)::
 
-    See Documentation/doc-guide/sphinx.rst.
+    See Documentation/doc-guide/sphinx.rst. This always works.
+    Take a look at sphinx.rst, which is at this same directory.
+    Read ../sphinx.rst, which is one directory above.
 
-If you want to use a relative path, you need to use Sphinx's ``doc`` directive.
-For example, referencing this page from the same directory would be done as::
+If you want the link to have a different rendered text other than the document's
+title, you need to use Sphinx's ``doc`` role. For example::
 
-    See :doc:`sphinx`.
+    See :doc:`my custom link text for document sphinx <sphinx>`.
+
+For most use cases, the former is preferred, as it is cleaner and more suited
+for people reading the source files. If you come across a ``:doc:`` usage that
+isn't adding any value, please feel free to convert it to just the document
+path.
 
 For information on cross-referencing to kernel-doc functions or types, see
 Documentation/doc-guide/kernel-doc.rst.