[wwwdocs] Generalize instructions and remove notes on repository mirroring via rsync.

[Joseph Myers]

On Sat, 18 Jan 2020, Gerald Pfeifer wrote:

> Should gcc-cvs really be listed there still?

Yes, it should.  The CVS repository was useful for fixing up cvs2svn 
branchpoints in the conversion to git; no doubt it may be useful for 
further fixes in any future move to any version control system that takes 
over from git in future.

-- 
Joseph S. Myers
joseph@codesourcery.com