[gcc r13-811] libbacktrace: update README

[gcc r13-811] libbacktrace: update README

[Ian Lance Taylor]

https://gcc.gnu.org/g:f535f9d7b05db6fa02e90c90173c597d1972faa1

commit r13-811-gf535f9d7b05db6fa02e90c90173c597d1972faa1
Author: Ian Lance Taylor <iant@golang.org>
Date:   Sat May 28 07:57:32 2022 -0700

    libbacktrace: update README
    
            PR libbacktrace/105721
            * README: Update.

Diff:
---
 libbacktrace/README | 32 ++++++++++++++++++++------------
 1 file changed, 20 insertions(+), 12 deletions(-)

diff --git a/libbacktrace/README b/libbacktrace/README
index e8b225745c9..6225f92b855 100644
--- a/libbacktrace/README
+++ b/libbacktrace/README
@@ -1,23 +1,31 @@
 The libbacktrace library
-Initially written by Ian Lance Taylor <iant@google.com>
+Initially written by Ian Lance Taylor <iant@golang.org>
 
 The libbacktrace library may be linked into a program or library and
-used to produce symbolic backtraces.  Sample uses would be to print a
-detailed backtrace when an error occurs or to gather detailed
-profiling information.
+used to produce symbolic backtraces.
+Sample uses would be to print a detailed backtrace when an error
+occurs or to gather detailed profiling information.
+In general the functions provided by this library are async-signal-safe,
+meaning that they may be safely called from a signal handler.
 
-The libbacktrace library is provided under a BSD license.  See the
-source files for the exact license text.
+The libbacktrace library is provided under a BSD license.
+See the source files for the exact license text.
 
 The public functions are declared and documented in the header file
 backtrace.h, which should be #include'd by a user of the library.
 
 Building libbacktrace will generate a file backtrace-supported.h,
 which a user of the library may use to determine whether backtraces
-will work.  See the source file backtrace-supported.h.in for the
-macros that it defines.
+will work.
+See the source file backtrace-supported.h.in for the macros that it
+defines.
 
-As of September 2012, libbacktrace only supports ELF executables with
-DWARF debugging information.  The library is written to make it
-straightforward to add support for other object file and debugging
-formats.
+As of October 2020, libbacktrace supports ELF, PE/COFF, Mach-O, and
+XCOFF executables with DWARF debugging information.
+In other words, it supports GNU/Linux, *BSD, macOS, Windows, and AIX.
+The library is written to make it straightforward to add support for
+other object file and debugging formats.
+
+The library relies on the C++ unwind API defined at
+https://itanium-cxx-abi.github.io/cxx-abi/abi-eh.html
+This API is provided by GCC and clang.