gmp 4.1 documentation improvement suggestions

[Paul Zimmermann]

   It's surprising that mpfr doesn't track precision. Its documentation
   seems to say that the main difference between mpfr_* and mpf_* is that
   mpfr knows the "actual exact number of precise bits" in any number. They
   use this wording in the manual. This seems to suggest that e.g.
   "10.010-10.000=0.0100" and the result will know that it has 2 precise
   bits. At least I thought so after reading the mpfr manual and after 
   glancing at the code for adding two numbers in mpfr_add. Maybe I was 
   mistaken.

mpfr is only an extension to multiple-precision of the IEEE 754 fixed
precision arithmetic. The mpfr documentation doesn't say that mpfr knows
the "actual exact number of precise bits" (otherwise please tell us where,
it is a mistake) but that whenever you ask for p bits of precision for a
mpfr variable, you get *exactly* p bits.

It is possible to build on mpfr a tool that keeps track of the precision of
computations, but this would be a different project.

Paul