[PATCH] SH: Add documents for -m2a* option

[Takashi Yoshii]

Hi,
> Thanks for the patch!  The ChangeLog entry should be
...
> ... Next time, please refer how your patch is tested.  ...
Sorry for my badly formed report.
I have update svn, and know it fixed on svn, thank you.

This time I tested by configuring with --enable-incomplete-targets,
make all-gcc; make install-gcc; make pdf. And then checked
 gcc.1, gcc.info, gcc.pdf and gcc --target-help
by my eyes, on sh4-linux-gnu system.
# It took days to do. I should have done it on PC...
/yoshii