Rollup merge of #127053 - xen0n:update-loong-docs, r=Nilstrieb,heiher

Update the LoongArch target documentation

The docs for the LoongArch targets are a bit dated since their introduction, and the prose has some room for improvement as well. Streamline a bit, referring to the neighboring targets' docs, and provide up-to-date information as much as I can come up with.

cc fellow target maintainer `@heiher` for review of target-specific bits

Author: matthiaskrgr