Revert "remove obsolete warning about fix rigid image flag restrictions"

This reverts commit 51e52b47.

doc/src/fix_spring.txt

@@ -89,7 +89,11 @@ NOTE: The center of mass of a group of atoms is calculated in
 group can straddle a periodic boundary.  See the "dump"_dump.html doc
 page for a discussion of unwrapped coordinates.  It also means that a
 spring connecting two groups or a group and the tether point can cross
-a periodic boundary and its length be calculated correctly.
+a periodic boundary and its length be calculated correctly.  One
+exception is for rigid bodies, which should not be used with the fix
+spring command, if the rigid body will cross a periodic boundary.
+This is because image flags for rigid bodies are used in a different
+way, as explained on the "fix rigid"_fix_rigid.html doc page.
 
 [Restart, fix_modify, output, run start/stop, minimize info:]