doc: add clarification to %sel()

Add clarification that %sel(), like %cond(), never expands
non-selected arguments.

Signed-off-by: H. Peter Anvin <[email protected]>

Author: H. Peter Anvin

doc/nasmdoc.src

@@ -2919,6 +2919,8 @@ expands to nothing.
 \c %define b 2
 \c %xdefine bstr %sel(b,"one","two","three") ; %define bstr "two"
 
+The arguments not selected are never expanded.
+
 
 \S{f_str} \i\c\{%str()} Function