[stringbuf.virtuals] Note in Returns: element that the return value for

non-error cases is specified in the Effects: element.

Fixes cplusplus#1552

Author: zygoloid

source/iostreams.tex

@@ -7571,6 +7571,7 @@
 
 \pnum
 \returns
+As specified above, or
 \tcode{traits::eof()}
 to indicate failure.
 
@@ -7624,6 +7625,7 @@
 
 \pnum
 \returns
+As specified above, or
 \tcode{traits::eof()}
 to indicate failure.