improve swtch comment

swtch.S

@@ -2,8 +2,9 @@
 #
 #   void swtch(struct context **old, struct context *new);
 # 
-# Save current register context in old
-# and then load register context from new.
+# Save the current registers on the stack, creating
+# a struct context, and save its address in *old.
+# Switch stacks to new and pop previously-saved registers.
 
 .globl swtch
 swtch: