[llvm-link] Add more detail to --internalize description

[maurer]

While the functionality of this flag is obvious in the implementation, tool users may not know what it does with the short description provided. Notably, it is not obvious from the short description that:

• Functions provided will be converted to internal linkage (and thus discarded if unused) even if unreferenced.
• Functions in the first file will not be internalized, even if referenced by a later one.

The Rust for Linux project has found use for this flag to support inlining static inline functions in C into code compiled by Rust when rustc and clang share a LLVM.

I wanted to send this PR both to:

1. Help others find the flag in the future
2. Clarify that our understanding of what we can rely upon this option to do is correct, and not just what the implementation happens to do at the moment.

cc @Darksonn (also with Rust-for-Linux)

cc @nikic Perhaps able to review or route us to an appropriate reviewer?

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[nikic]

I'd suggest adding more detailed documentation to the man page (https://github.com/llvm/llvm-project/blob/main/llvm/docs/CommandGuide/llvm-link.rst) rather than the option description.

[nikic]

I believe your description of the behavior is correct and I wouldn't expect it to change, but I've added a few more people to double check, as I'm not particularly familiar with llvm-link.

[teresajohnson]

I'm not an llvm-link user, but it does look like I reviewed @JDevlieghere 's change many years ago to add this functionality (https://reviews.llvm.org/D30738). This change is largely accurate, but in fact looking at that old patch the point was to internalize all but some special symbols, and symbols referenced by llvm*.used. That's probably a bit wordy to add to the option description, but you might want to note that it isn't all symbols.

I agree with @nikic 's suggestion to add documentation to the man page as well.