Extend docs (see #1393)

l3kernel/l3tl-build.dtx

@@ -62,8 +62,10 @@
 % Whilst the exact performance difference is dependent on the size of each
 % added block of tokens and the total number of blocks, in general, the
 % \cs[no-index]{tl_build_(g)put...} functions will out-perform the basic
-% \cs[no-index]{tl_(g)put...} equivalent if more than 200 non-empty addition
-% operations occur.
+% \cs[no-index]{tl_(g)put...} equivalent if more than 100 non-empty addition
+% operations occur. See
+% \url{https://github.com/latex3/latex3/issues/1393#issuecomment-1880164756}
+% for a more detailed analysis.
 %
 % \begin{function}[added = 2018-04-01]{\tl_build_begin:N, \tl_build_gbegin:N}
 %   \begin{syntax}