docs: clarify htaccess.RewriteBase is a backend path, not public URL

The previous description implied the value should mirror the public URL,
causing users behind a reverse proxy to misconfigure the setting and hit
infinite redirect loops. The value is an Apache RewriteBase directive
and must reflect where Nextcloud is served on the backend (relative to
DocumentRoot), not the public-facing URL prefix.

Split into "Direct setup" and "Reverse proxy" sections with a concrete
example of each, and added a matching note in the Pretty URLs section of
the source installation guide.

Fixes #13428

Signed-off-by: John Molakvoæ (skjnldsv) <skjnldsv@protonmail.com>

Author: skjnldsv

admin_manual/installation/source_installation.rst

@@ -191,8 +191,18 @@ if your setup is available on ``https://example.org/nextcloud`` or::
     'overwrite.cli.url' => 'https://example.org/',
     'htaccess.RewriteBase' => '/',
 
-if it isn't installed in a subfolder. Finally run this occ-command to update
-your .htaccess file::
+if it isn't installed in a subfolder.
+
+.. note::
+
+   ``htaccess.RewriteBase`` must match the path relative to Apache's DocumentRoot
+   where Nextcloud is served on the backend, not the public URL prefix. In a direct
+   Apache setup these are identical. Behind a reverse proxy that strips the URL
+   prefix — for example ``https://domain.com/nextcloud/`` forwarded to
+   ``http://localhost:8080/`` — the correct value is ``/`` even though the public
+   URL contains ``/nextcloud``.
+
+Finally run this occ-command to update your ``.htaccess`` file::
 
     sudo -E -u www-data php /var/www/nextcloud/occ maintenance:update:htaccess