[clang] Document -femit-compact-unwind option in the User’s Manual

[int3]

@davide just adding you in case you have opinions on the wording around the bincompat issue

[MaskRay]

clang/docs/UsersManual.rst

848

I think flag applies to these boolean options. This is an option, not a flag.

[int3]

option

[thakis]

Thanks!

clang/docs/UsersManual.rst

853

nit: I think most things in apple-land call it arm64, not aarch64

clang/docs/UsersManual.rst

 most filenames can be written to the file without any special formatting.
 Different Make tools will treat different sets of characters as "special"
 and use different conventions for telling the Make tool that the character
 is actually part of the filename. Normally Clang uses backslash to "escape"
 a special character, which is the convention used by GNU Make. The -MV
 option tells Clang to put double-quotes around the entire filename, which
 is the convention used by NMake and Jom.
 
+.. option:: -femit-dwarf-unwind=<value>
+
+  When to emit DWARF unwind (EH frame) info. This is a Mach-O-specific option.
+
+  Valid values are:
+
+  * ``no-compact-unwind`` - Only emit DWARF unwind when compact unwind encodings
+    aren't available. This is the default for arm64.
+  * ``always`` - Always emit DWARF unwind regardless.
+  * ``default`` - Use the platform-specific default (``always`` for all
+    non-arm64-platforms).
+
+``no-compact-unwind`` is a performance optimization -- Clang will emit smaller
+object files that are more quickly processed by the linker. This may cause
+binary compatibility issues on older x86_64 targets, however, so use it with
+caution.
+
 .. _configuration-files:
 
 Configuration files
 -------------------
 
 Configuration files group command-line options and allow all of them to be
 specified just by referencing the configuration file. They may be used, for
 example, to collect options required to tune compilation for particular