[docs] Fix self-contradictory description of llvm_unreachable

Just two paragraphs above it says:
"If the compiler does not support this [skipping code generation for a particular branch], it will fall back
to the "abort" implementation."

And that actually correctly describes llvm_unreachable implementation.

Differential Revision: https://reviews.llvm.org/D81130

llvm/docs/CodingStandards.rst

@@ -1200,8 +1200,7 @@ unconditionally a bug (not originating from user input; see below) of some kind.
 Use of ``assert`` should always include a testable predicate (as opposed to
 ``assert(false)``).
 
-Neither assertions or ``llvm_unreachable`` will abort the program on a release
-build. If the error condition can be triggered by user input then the
+If the error condition can be triggered by user input then the
 recoverable error mechanism described in :doc:`ProgrammersManual` should be
 used instead. In cases where this is not practical, ``report_fatal_error`` may
 be used.

llvm/include/llvm/Support/ErrorHandling.h

@@ -127,7 +127,7 @@ llvm_unreachable_internal(const char *msg = nullptr, const char *file = nullptr,
 /// In !NDEBUG builds, prints the message and location info to stderr.
 /// In NDEBUG builds, becomes an optimizer hint that the current location
 /// is not supposed to be reachable.  On compilers that don't support
-/// such hints, prints a reduced message instead.
+/// such hints, prints a reduced message instead and aborts the program.
 ///
 /// Use this instead of assert(0).  It conveys intent more clearly and
 /// allows compilers to omit some unnecessary code.