sphinx warnings in 1.1-maint branch

[ThomasWaldmann]

(borg-env) tw@mba2020 borg % cd docs
(borg-env) tw@mba2020 docs % make html
sphinx-build -b html -d _build/doctrees   . _build/html
Running Sphinx v4.0.3
loading pickled environment... done
building [mo]: targets for 0 po files that are out of date
building [html]: targets for 23 source files that are out of date
updating environment: 0 added, 38 changed, 0 removed
reading sources... [100%] usage/serve                                                                                                                                       

../AUTHORS:1: WARNING: Unknown target name: "support section".
/Users/tw/w/borg/docs/installation.rst:78: WARNING: Duplicate explicit target name: "homebrew".
/Users/tw/w/borg/docs/installation.rst:51: WARNING: Duplicate target name, cannot be used as a unique reference: "homebrew".
/Users/tw/w/borg/docs/installation.rst:235: WARNING: Duplicate target name, cannot be used as a unique reference: "homebrew".
looking for now-outdated files... none found
pickling environment... done
checking consistency... /Users/tw/w/borg/docs/man_intro.rst: WARNING: document isn't included in any toctree