Doc fixups

[Rot127]

Your checklist for this pull request

• I've documented or updated the documentation of every API function and struct this PR changes.
• I've added tests that prove my fix is effective or that my feature works (if possible)

Detailed description

Corrections of the docs @moste00 found during refactoring RISCV

• Describe ArchLinkage.h in more detail.
• Add hint about MCUpdate that the Assembler can be used as well if not tests generate.

Test plan

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Closing issues

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[moste00]

@Rot127 should there an implied relationship between the vendor/llvm_root and the capstone root, or are they indepedent directories on the same level?

[Rot127]

Good catch.

The dir to clone LLVM into is:
suite/auto-sync/vendor/

llvm_root is the directory. Of course you can clone LLVM somewhere else and just add an llvm_root link

This should be made clearer.