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.
general.rst and man_intro.rst both included usage_general.rst.inc, which resulted in three Sphinx warning "WARNING: duplicate label".
To prevent this we move all sections of usage_general into own include files and add a second usage_general file without the three labels.
