* Clarify that GMP and intDsize cannot be tuned separately.

src/TUNING

@@ -1,12 +1,14 @@
 Tips for performance tuning on a specific architecture:
 
-1. Choose the optimal limb size (intDsize). This is fundamental. On 32-bit
+1a. Choose the optimal digit size (intDsize). This is fundamental. On 32-bit
     platforms intDsize=32 is best. On 64-bit platforms intDsize=64 may be
     better, especially if there is a 64x64-bit multiplication in hardware.
 
-2. Tune GMP.
+1b. Alternatively, tune GMP. When GMP is used, CLN's digit size (intDsize) has
+    to match GMP's limb size (sizeof(mp_limb_t)). There is nothing to do at the
+    CLN side: The configure script will take care of intDsize automatically.
 
-3. The break-even points between several algorithms for the same task
+2.  The break-even points between several algorithms for the same task
     have to be determined experimentally, in the order given below:
 
     multiplication: