rephrase some warnings, fixes#5164
borg check --repair and borg recreate are now present in the code since rather long, so they are not experimental any more.
borg recreate might be used wrongly (e.g. accidentally excluding everything / not matching anything when recreating an archive). added some warning words in the docs, but it will not ask for confirmation any more.
borg check: there might be kinds of corruption borg check --repair can not fix and it might make things even worse while trying to fix. so this will still ask for confirmation, just with different wording.
Because some of the JSON blocks in frontends.rst included non-JSON
contents (user replies, etc.) the blocks didn't parse/highlight and
`make html` complained:
borg/docs/internals/frontends.rst:28: WARNING: Could not lex literal_block as "json".
There's no easy way to enable and disable highlighting just for
specific lines, but individually unsetting the highlight language per
block suppresses the warnings.
borg list by default does not compute expensive values except when
they are needed. whether they are needed is determined by the format,
in standard mode as well as in --json mode.
When opening a repository, always try to read the magic number of the
latest segment and compare it to the Attic segment magic (unless the
repository is opened for upgrading). If an Attic segment is detected,
raise a dedicated exception, telling the user to upgrade the repository
first.
Fixes#1933.
