AH01797: client denied by server configuration: /usr/share/doc

Solution 1:

In 2.2, access control based on client hostname, IP address, and other characteristics of client requests was done using the directives Order, Allow, Deny, and Satisfy.

In 2.4, such access control is done in the same way as other authorization checks, using the new module mod_authz_host. The old access control idioms should be replaced by the new authentication mechanisms, although for compatibility with old configurations, the new module mod_access_compat is provided.

Looks like you've already set the new Require directive, so just remove the deprecated access directives and run sudo service apache2 reload

Solution 2:

Since some time has passed without any answer, I decided to (partially) answer my own question according to my research so far.

1. Unfortunately, the question why Googlebot and Baiduspider are trying to access the Apache documentation through my server remains unanswered.
2. The /manual/... URLs get mapped to /usr/share/doc/apache2-doc/manual/... thanks to a pre-installed Alias on Ubuntu: I guess that this is so, in order to make it convenient to access the documentation. In my case this is not needed, so I decided to remove the Alias by issuing a2disconf apache2-doc followed by service apache2 reload.
3. There is no reason to regard the log entries as signs of misconfiguration, as they are rather the desired behaviour. Before removing the Alias, the access to the documentation was blocked by the vhost config, thus returning a 403 "Forbidden" status code. After removing the Alias, the server correctly returns a 404 "Not Found" status code.