other/10301: Side effects of architecture specific flags are not documented.

[Garen]

http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=10301

The part I was referring to inparticular was what the -march settings turn on.  
For example, -march=pentium4 turns on -msse2.  All of those things aren't 
mentioned anywhere in the manual:

http://gcc.gnu.org/onlinedocs/gcc-3.2.2/gcc/i386-and-x86-64-Options.html#i386%20and%20x86-64%20Options