doc: explain how to use custom lexers in the latest minted (#3047)

v3.0 is not yet released; in the meantime, the previous instructions did
not work in the latest version without some hacks.
[Zulip
thread](https://leanprover.zulipchat.com/#narrow/stream/113489-new-members/topic/XeLaTeX.20with.20minted.20error/near/406959183)

doc/syntax_highlight_in_latex.md

@@ -67,6 +67,9 @@ theorem funext {f₁ f₂ : ∀ (x : α), β x} (h : ∀ x, f₁ x = f₂ x) : f
 \end{document}
 ```
 
+If your version of `minted` is v2.7 or newer, but before v3.0,
+you will additionally need to follow the workaround described in https://github.com/gpoore/minted/issues/360.
+
 You can then compile `test.tex` by executing the following command:
 
 ```bash