[llvm-objcopy] Improve help messages (#82830)

https://reviews.llvm.org/D63820 added
llvm/docs/CommandGuide/llvm-objcopy.rst with clearer semantics, e.g.

```
Read a list of names from the file <filename> and mark defined symbols with those names as global in the output

instead of the help message
Read a list of symbols from <filename> and marks them global" (omits "defined")

Rename sections called <old> to <new> in the output

instead of the help message
Rename a section from old to new (multiple sections may be named <old>
```

Sync the help messages to incorporate the CommandGuide improvement.

While here, switch to the conventional imperative sentences for a few
options. Additionally, mark some options as grp_coff or grp_macho.