Add a note about split bib options while we are at it (#1308)

doc/latex/biblatex/biblatex.tex

@@ -3877,6 +3877,13 @@ \subsubsection{The Bibliography}
 
 Filter the entries with check \prm{name}, as defined with \cmd{defbibcheck}. See \secref{use:bib:flt} for details.
 
+\end{optionlist*}
+
+The following options are useful in <split bibliography> setups, where several bibliographies (with different filtering options)
+are printed in the same document.
+
+\begin{optionlist*}
+
 \valitem{resetnumbers}{true,false,number}
 
 This option applies to numerical citation\slash bibliography styles only and requires that the \opt{defernumbers} option from \secref{use:opt:pre:gen} be enabled globally. If enabled, it will reset the numerical labels assigned to the entries in the respective bibliography, \ie the numbering will restart at~1. You can also pass a number to this option, for example: \texttt{resetnumbers=10} to reset numbering to the specified number to aid numbering continuity across documents. Use this option with care as \biblatex can not guarantee unique labels globally if they are reset manually.