Trim some redundant and a bit of misleading text:

Trim some redundant and a bit of misleading text:
1. The values of #define'd constants are part of the ABI, but not of the API.
Programmers need not worry about them and must not rely on them.
Consequently, do not list these values in the SYNOPSIS.
2. libevent can be used for any event loop, not only for the main loop
of a program.
3. No need to state numbers of arguments, they are obvious from the SYNOPSIS.
4. If a function is documented to require a pointer to a specific kind of
object as an argument, there is no need to say passing NULL is undefined.
OK jmc@ and Ted Bullock