[Bug target/109456] `-ffixed-reg` cannot prevent using `reg` for ABI-mandated roles (argument register etc) and the behavior should be documented more clearly

[gccriscvuser at proton dot me via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109456

--- Comment #12 from gccriscvuser at proton dot me ---
Updating the documentation is good, but there should also be an error
diagnostic, right?