The src pages of a cargo typstdoc build show the rendering, not the comments

[luiswirth]

rustdoc writes its source pages from the file it was given,
and cargo typstdoc gives it a copy whose doc comments have become
#[doc = "..."] attributes holding MathML.
The line numbers match, so every link lands where it should,
but the source a reader sees is the rendering rather than what was written.

Nothing in the doc pages themselves is affected.

Ways out, none tried:

• Two rustdoc runs, the source pages from the one over the original sources.
• Keep the comments and add the rendering, which needs a place for the second
  to live that rustdoc reads and the first does not.
• --html-no-source, which drops the pages rather than fixing them.