Strengthen smb backend's PHP smbclient recommendation (#9428)

Updates the prerequisite note in admin_manual/.../smb.rst to:

- name the actual package (php-smbclient / libsmbclient-php) so
  admins know what to apt/yum install;
- explain *why* the PHP module is preferred (without it the backend
  falls back to the smbclient binary, which can fail to download
  files larger than ~512 MB from external SMB shares — the bug
  reported in nextcloud/server#31308);
- link to that upstream issue so admins hitting the failure mode
  can find this advice from the issue tracker.

Author: mvanhorn

admin_manual/configuration_files/external_storage/smb.rst

@@ -6,9 +6,14 @@ Nextcloud can connect to Windows file servers or other SMB-compatible servers
 with the SMB/CIFS backend.
 
 .. note:: The SMB/CIFS backend requires ``smbclient`` or
-   the PHP smbclient module to be installed on the Nextcloud server. The PHP
-   smbclient module is preferred, but either will work. These
-   should be included in any Linux distribution. (See `PECL smbclient
+   the PHP smbclient module (``php-smbclient`` / ``libsmbclient-php``) to be
+   installed on the Nextcloud server. The PHP smbclient module is **strongly
+   preferred** — without it, the backend falls back to spawning the
+   ``smbclient`` binary, which has known limitations (notably, downloads of
+   files larger than ~512 MB from external SMB shares can fail; see
+   `nextcloud/server#31308
+   <https://github.com/nextcloud/server/issues/31308>`_). These should be
+   included in any Linux distribution. (See `PECL smbclient
    <https://pecl.php.net/package/smbclient>`_ if your distro does not include
    them.)