[rand.util.seedseq] Add cross-reference for "writable".

Co-Authored-By: Johel Ernesto Guerrero Peña <[email protected]>

Author: zygoloid

source/numerics.tex

@@ -4395,7 +4395,7 @@
 \begin{itemdescr}
 \pnum
 \mandates
-  Values of type \tcode{result_type} are writable to \tcode{dest}.
+  Values of type \tcode{result_type} are writable\iref{iterator.requirements.general} to \tcode{dest}.
 
 \pnum
 \expects