[LangRef] Document that sret only works with void returns (llvm#112167)

Author: workingjubilee

llvm/docs/LangRef.rst

@@ -1309,12 +1309,14 @@ Currently, only the following parameter attributes are defined:
     structure that is the return value of the function in the source
     program. This pointer must be guaranteed by the caller to be valid:
     loads and stores to the structure may be assumed by the callee not
-    to trap and to be properly aligned. This is not a valid attribute
-    for return values.
+    to trap and to be properly aligned.
 
     The sret type argument specifies the in memory type, which must be
     the same as the pointee type of the argument.
 
+    A function that accepts an ``sret`` argument must return ``void``.
+    A return value may not be ``sret``.
+
 .. _attr_elementtype:
 
 ``elementtype(<ty>)``