update documentation regarding fp_isprime()

Author: sjaeckel

tfm.tex

@@ -412,12 +412,17 @@ \section{Prime Numbers}
 To quickly test a number for primality call this function.
 
 \index{fp\_isprime}
+\index{fp\_isprime\_ex}
 \begin{verbatim}
-int fp_isprime(fp_int *a);
+int fp_isprime_ex(fp_int *a, int t);
 \end{verbatim}
 This will return \textbf{FP\_YES} if $a$ is probably prime.  It uses 256 trial divisions and
-eight rounds of Rabin-Miller testing.  Note that this routine performs modular exponentiations
+$t$ rounds of Rabin-Miller testing.  Note that this routine performs modular exponentiations
 which means that $a$ must be in a valid range of precision.
+The valid range of $t$ is $1 \ldots 256$.
+
+For backwards compatibility reasons the API function fp\_isprime(a) is still available
+and simply calls fp\_isprime\_ex(a, 8).
 
 \chapter{Porting TomsFastMath}
 \label{chap:asmops}