doc: improve intprops doc * doc/intprops.texi: Tighten up wording, by saying that macros "yield 1 if X, 0 otherwise" rather than the weaker "yield 1 if X". Say "yield" rather than "return" since the macros are not functions. Say "1" and "0" rather than "true" and "false" since the macros yield int. Say that stdckdint.h is the standard alternative to the _WRAPV macros. Mention another source of problems with the _OVERFLOW macros.