* Document _URL/_SAVE_AS path matching req. Fix #2158

This commit addresses the fact that some URLs (primarily {attach}) are based
on the *_SAVE_AS path instead of the *_URL path. It updates the documentation
to point out that the paths of corresponding _URL and _SAVE_AS settings should
match, and that the OUTPUT directory should correspond to the web server's
document root, in order to avoid producing these incorrect URLs.

* Clarify {attach} and *_URL / *_SAVE_AS caveat

Co-authored-by: Justin Mayer <entroP@gmail.com>

Test updates

Reduce warnings and errors in sample content

This reduces the warnings and errors when generating the sample content
from twelve to one (intentional) warning. The latter is expected by
assertLogCountEqual() in the "test_basic_generation_works" test.