[except.terminate] Better describe the function

While 'std:terminate' was originally conceived as the way to
report failures in the exception handling machinery, it has
evolved to become a more general tool for reporting unrecoverable
failures in the C++ runtime.  This rewording attempts to address
that evolving design, and in doing so addresses the outstanding
%FIXME% that the current text is not adequately descriptive in
the first place.

source/exceptions.tex

@@ -1016,9 +1016,9 @@
 
 \pnum
 \indextext{\idxcode{terminate}}%
-% FIXME: What does it mean to abandon exception handling?
-In some situations, exception handling is abandoned
-for less subtle error handling techniques.
+Some errors in a program cannot be recovered from, such as when an exception is
+not handled or a thread is destroyed while still running. In such cases,
+the function \tcode{std::terminate}\iref{exception.terminate} is invoked.
 \begin{note}
 These situations are:
 \indextext{\idxcode{terminate}!called}%
@@ -1112,8 +1112,6 @@
 
 \pnum
 \indextext{\idxcode{terminate}}%
-In such cases,
-the function \tcode{std::terminate} is invoked\iref{exception.terminate}.
 In the situation where no matching handler is found, it is
 \impldef{stack unwinding before invocation of \tcode{std::terminate}}
 whether or not the stack is unwound