intprops: TYPE_IS_INTEGER, TYPE_SIGNED not integer constant exprs

intprops: TYPE_IS_INTEGER, TYPE_SIGNED not integer constant exprs

* doc/intprops.texi (Integer Type Determination): Fix
documentation for TYPE_IS_INTEGER: it returns an constant
expression, not an integer constant expression.  Fix doc for
TYPE_SIGNED: it returns an integer constant expression only if its
argument is an integer type.  (TYPE_IS_INTEGER is the same, but is
hardly worth documented that way....)
(cherry picked from commit 48cc7484c740d7f0e84fccd1fde11059986e3b94)