strtod.3: Fix return value for underflow

In C89 strtod returns zero on underflow, but since C99 it can return non-zero. This means the strtod.3 page contradicts all recent C and POSIX standards. Both C and POSIX say "smallest normalized positive number", but for consistency with HUGE_VAL, HUGE_VALF and HUGE_VALL this patch uses the constants for those numbers.

Also slightly improve the presentation of return values for overflow.

man3/strtod.3 +14 -6

index a271498d5..56ddb5f27 100644
@@ -138,15 +138,23 @@ is stored in the location referenced by
  .IR endptr .
  .PP
  If the correct value would cause overflow, plus or minus
-.B HUGE_VAL
-.RB ( HUGE_VALF ,
-.BR HUGE_VALL )
-is returned (according to the sign of the value), and
+.BR HUGE_VAL ,
+.BR HUGE_VALF ,
+or
+.B HUGE_VALL
+is returned (according to the return type and sign of the value),
+and
  .B ERANGE
  is stored in
  .IR errno .
-If the correct value would cause underflow, zero is
-returned and
+.PP
+If the correct value would cause underflow,
+a value with magnitude no larger than
+.BR DBL_MIN ,
+.BR FLT_MIN ,
+or
+.B LDBL_MIN
+is returned and
  .B ERANGE
  is stored in
  .IR errno .