other/8947: Please add a Warning about "-malign-double" in docs

[Janis Johnson]

The mainline version of install.texi includes the following
after the description of -malign-double:

  @strong{Warning:} if you use the @samp{-malign-double}
  switch, structures containing the above types will be
  aligned differently than the published application binary
  interface specifications for the 386.

If that's not strong enough, we could also add:

  Code compiled with @option{-malign-double} is not binary
  compatible with code compiled without that switch.

Similar language is now used for warnings about several
other options.

http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=8947