patch 8.2.3917: the eval.txt help file is way too big

Problem:    The eval.txt help file is way too big.
Solution:   Move the builtin function details to a separate file.
diff --git a/runtime/doc/Makefile b/runtime/doc/Makefile
index e0c0f84..de986a6 100644
--- a/runtime/doc/Makefile
+++ b/runtime/doc/Makefile
@@ -16,6 +16,7 @@
 DOCS = \
 	arabic.txt \
 	autocmd.txt \
+	builtin.txt \
 	change.txt \
 	channel.txt \
 	cmdline.txt \
@@ -161,6 +162,7 @@
 HTMLS = \
 	arabic.html \
 	autocmd.html \
+	builtin.html \
 	change.html \
 	channel.html \
 	cmdline.html \