[Backtracing][Docs] Add some information about signal handlers for macOS.

Added a list of handled signals and some notes on what the runtime will
do if it finds signal handlers already configured.

rdar://105394365

Author: al45tair

docs/Backtracing.rst

@@ -167,3 +167,57 @@ require memory allocation, fetching and reading remote files and so on).
 You shouldn't try to run ``swift-backtrace`` yourself; it has unusual
 requirements, which vary from platform to platform.  Instead, it will be
 triggered automatically by the runtime.
+
+System specifics
+----------------
+
+macOS
+^^^^^
+
+On macOS, we catch crashes and other events using a signal handler.  At time of
+writing, this is installed for the following signals:
+
++--------------+--------------------------+-------------------------------------+
+| Signal       | Description              | Comment                             |
++====+=========+==========================+=====================================+
+|  3 | SIGQUIT | Quit program             |                                     |
++----+---------+--------------------------+-------------------------------------+
+|  4 | SIGILL  | Illegal instruction      |                                     |
++----+---------+--------------------------+-------------------------------------+
+|  5 | SIGTRAP | Trace trap               |                                     |
++----+---------+--------------------------+-------------------------------------+
+|  6 | SIGABRT | Abort program            |                                     |
++----+---------+--------------------------+-------------------------------------+
+|  8 | SIGFPE  | Floating point exception | On Intel, integer divide by zero    |
+|    |         |                          | also triggers this.                 |
++----+---------+--------------------------+-------------------------------------+
+| 10 | SIGBUS  | Bus error                |                                     |
++----+---------+--------------------------+-------------------------------------+
+| 11 | SIGSEGV | Segmentation violation   |                                     |
++----+---------+--------------------------+-------------------------------------+
+
+If crash catching is enabled, the signal handler will be installed for any
+process that links the Swift runtime.  If you replace the handlers for any of
+these signals, your program will no longer produce backtraces for program
+failures that lead to the handler you have replaced.
+
+Additionally, the runtime will configure an alternate signal handling stack, so
+that stack overflows can be successfully trapped.
+
+Note that the runtime will not install its signal handlers for a signal if it
+finds that there is already a handler for that signal.  Similarly if something
+else has already configured an alternate signal stack, it will leave that
+stack alone.
+
+Once the backtracer has finished handling the crash, it will allow the crashing
+program to continue and crash normally, which will result in the usual Crash
+Reporter log file being generated.
+
+Crash catching *cannot* be enabled for setuid binaries.  This is intentional as
+doing so might create a security hole.
+
+Other Darwin (iOS, tvOS)
+^^^^^^^^^^^^^^^^^^^^^^^^
+
+Crash catching is not enabled for non-macOS Darwin.  You should continue to look
+at the system-provided crash logs.