[llvm] [doc][RISCV] Add documentation for customizing VCIX scheduling info (PR #183129)

[Min-Yih Hsu via llvm-commits]

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+===================================================
+Scheduling Information for RISC-V VCIX Instructions
+===================================================
+
+.. contents::
+   :local:
+
+Summary
+-------
+The purpose of this document is to outline how the scheduling information for RISC-V's ``XSfvcp`` extension -- SiFive Vector Coprocessor Interface (VCIX) -- in LLVM works, why it works the way it does, and how one may modify the code to support their VCIX needs.
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mshockwave wrote:

I thought we're lifting this restriction for documentation: https://discourse.llvm.org/t/rfc-remove-80-column-limit-in-documentation-files/89678

https://github.com/llvm/llvm-project/pull/183129