[docs][llvm-objcopy] Add description of binary input/output to doc

[jhenderson]

Summary

We briefly referred to being able to specify --target=binary without explaining what binary input/output meant. This change adds a section on this.

[abrachet]

LGTM

docs/CommandGuide/llvm-objcopy.rst

466 ↗

(On Diff #208162)

This is super nit, and I'm not sure if either way is correct. But it seems like known option arguments use one back tick and then the user specified flags like <filename> use two. See --add-symbol on line 170 and --compress-debug-sections on line 221. If we follow those then 'binary' should only be with single back ticks. Of course line 460 uses the double back ticks though. I don't think it makes a huge difference either way just thought I would bring this up.

[jhenderson]

docs/CommandGuide/llvm-objcopy.rst

466 ↗

(On Diff #208162)

The difference is in rendering. Double back-tick makes it monospace, whilst single back-tick makes it italicised (and possibly monospace, though it's harder to tell). Looking back at what I did previously, it looks like I used single back ticks for valid values everywhere, so it probably makes sense to update here and line 460 before committing. Thanks!

llvm/trunk/docs/CommandGuide/llvm-objcopy.rst

 - `elf32-ntradlittlemips`
 - `elf32-tradbigmips`
 - `elf32-tradlittlemips`
 - `elf64-tradbigmips`
 - `elf64-tradlittlemips`
 - `elf32-sparc`
 - `elf32-sparcel`
 
-Additionally, all targets except ``binary`` and ``ihex`` can have ``-freebsd``
-as a suffix.
+Additionally, all targets except `binary` and `ihex` can have `-freebsd` as a
+suffix.
+
+BINARY INPUT AND OUTPUT
+-----------------------
+
+If `binary` is used as the value for :option:`--input-target`, the input file
+will be embedded as a data section in an ELF relocatable object, with symbols
+``_binary_<file_name>_start``, ``_binary_<file_name>_end``, and
+``_binary_<file_name>_size`` representing the start, end and size of the data,
+where ``<file_name>`` is the path of the input file as specified on the command
+line with non-alphanumeric characters converted to ``_``.
+
+If `binary` is used as the value for :option:`--output-target`, the output file
+will be a raw binary file, containing the memory image of the input file.
+Symbols and relocation information will be discarded. The image will start at
+the address of the first loadable section in the output.
 
 EXIT STATUS
 -----------
 
 :program:`llvm-objcopy` exits with a non-zero exit code if there is an error.
 Otherwise, it exits with code 0.
 
 BUGS