This sentence is precisely what I feel is missing in the documentation.
Don't you think? Could you add this information to the manual page?
How about this?
diff --git a/tmac/groff_man.7.man.in b/tmac/groff_man.7.man.in
index 34a14c8c7..8416825cf 100644
--- a/tmac/groff_man.7.man.in
+++ b/tmac/groff_man.7.man.in
@@ -1980,9 +1980,14 @@ or
or its synonyms is called;
these clear the indentation entirely.
.
-Insets created by
+A relative inset created by
.B .RS
-move the left margin and persist until
+saves the current indentation,
+resets it to zero,
+and
+moves the left margin.
+.
+The new left margin persists until
.BR .RS ,
.BR .RE ,
.BR .SH ,