[RFC] Update Stage 4 description

[Tom de Vries]

[ To revisit https://gcc.gnu.org/ml/gcc-patches/2018-04/msg00385.html ]

The current formulation for the description of Stage 4 here (
https://gcc.gnu.org/develop.html ) is:
...
During this period, the only (non-documentation) changes that may be
made are changes that fix regressions.

Other changes may not be done during this period.

Note that the same constraints apply to release branches.

This period lasts until stage 1 opens for the next release.
...

This updated formulation was proposed by Richi (with a request for
review of wording):
...
 During this period, the only (non-documentation) changes that may
 be made are changes that fix regressions.

-Other changes may not be done during this period.
+Other important bugs like wrong-code, rejects-valid or build issues may
+be fixed as well.  All changes during this period should be done with
+extra care on not introducing new regressions - fixing bugs at all cost
+is not wanted.

 Note that the same constraints apply to release branches.

 This period lasts until stage 1 opens for the next release.
...

If a text can be agreed upon, then I can prepare a patch for wwwdocs.

Thanks,
- Tom