Wrong path for attached static content urls (#2158)

* 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>

docs/settings.rst

@@ -168,7 +168,8 @@ Basic settings
 
 .. data:: OUTPUT_PATH = 'output/'
 
-   Where to output the generated files.
+   Where to output the generated files. This should correspond to your web
+   server's virtual host root directory.
 
 .. data:: PATH
 
@@ -358,6 +359,7 @@ Basic settings
 
    The IP to which to bind the HTTP server.
 
+.. _url-settings:
 
 URL settings
 ============
@@ -378,6 +380,12 @@ variables allow you to place your articles in a location such as
 example below). These settings give you the flexibility to place your articles
 and pages anywhere you want.
 
+.. note::
+    If a ``*_SAVE_AS`` setting contains a parent directory that doesn't match
+    the parent directory inside the corresponding ``*_URL`` setting, this may
+    cause Pelican to generate unexpected URLs in a few cases, such as when
+    using the ``{attach}`` syntax.
+
 If you don't want that flexibility and instead prefer that your generated
 output paths mirror your source content's filesystem path hierarchy, try the
 following settings::