Fix two documentation bugs:

Fix two documentation bugs:
1. The function event_base_free(3) does reset the global variable
to the initial state when needed.
2. The CAVEATS section got the issue backwards: multiple threads
per base cause trouble, whereas multiples bases per thread do not.

While here, also avoid the plural when talking about the global
variable used by event_init(3): there is only one.

Using input from jmc@ and from Ted Bullock.