RISC-V: Correct and improve the "-mabi" documentation

RISC-V: Correct and improve the "-mabi" documentation

The documentation for the "-mabi" argument on RISC-V was incorrect.  We
chose to treat this as a documentation bug rather than a code bug, and
to make the documentation match what GCC currently does.  In the
process, I also improved the documentation a bit.

Thanks to Alex Bradbury for finding the bug!

PR target/82717: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82717

gcc/ChangeLog

2017-10-27  Palmer Dabbelt  <palmer@dabbelt.com>

        PR target/82717
        * doc/invoke.texi (RISC-V) <-mabi>: Correct and improve.

From-SVN: r254153