Fix #9529: LaTeX: named footnotes are converted to "?"

[tk0miya]

Feature or Bugfix

• Bugfix

Purpose

• Named auto numbered footnote (ex. [#named]) that is referred
  multiple times was rendered to a question mark.
• This calls \sphinxstepexplicit for every footnote node that is
  referred multiple times.
• refs: sphinxfootnotemark is not working correctly after update sphinx >=3.5.0 #9529
• including test: Add testcase for named footnotes #10168

[jfbu]

Indeed you are right that what was needed was to know which footnotes are multiply referred too, so LGTM!

small nits: The macro \sphinxstepexplicit is now a bit misnamed, let's however stick with its current name. But at some point a comment in texinputs/sphinx.sty may need update like this:

index 4d42199a7..6c9f1606e 100644
--- a/sphinx/texinputs/sphinx.sty
+++ b/sphinx/texinputs/sphinx.sty
@@ -300,9 +300,9 @@
 % Support scopes for footnote numbering
 \newcounter{sphinxscope}
 \newcommand{\sphinxstepscope}{\stepcounter{sphinxscope}}
-% Explicitly numbered footnotes may be referred to, and for this to be
-% clickable we need to have only one target. So we will step this at each
-% explicit footnote and let \thesphinxscope take it into account
+% Some footnotes are multiply referred-to.  For unique hypertarget in pdf,
+% we need an additional counter.  It is called "sphinxexplicit" for legacy
+% reasons as "explicitly" numbered footnotes may be multiply referred-to.
 \newcounter{sphinxexplicit}
 \newcommand{\sphinxstepexplicit}{\stepcounter{sphinxexplicit}}
 % Some babel/polyglossia languages fiddle with \@arabic, so let's be extra

This extra patch may be postponed to later occaion.

edited: what follows was based on some confused memory, the situation described below with multiple sources may still lead to wrong hyperlinks even with this PR (related #10175).

I have noticed that this PR gives correct hyperlinks even in situation where named footnotes originate in multiple documents, and so may use for example both 1 as footnote mark, which in pdf single document output may be confusing if on same page, but at least with this PR the hyperlink is ok. For example

========== ===
Test [#a]_
========== ===
   1       One
========== ===

.. [#a] Footnote one.

in document doc1.rst and

========== ===
Test [#a]_
========== ===
   1       Two
========== ===

.. [#a] Footnote two.

in doc2.rst, both in toctree of index.rst. In future there is potential using the sphinxscope apparatus in sphinx.sty to create in PDF a disambiguating footnote mark, but at any rate already hyperlink will point to correct footnote, even if on same page in PDF.

[tk0miya]

Thank you for reviewing. Updated the comment now.

Let's discuss the problem with multiple sources in #10175.

Thanks as always!