meson_options.txt: disable the "smbclient" plugin by default

The bug https://bugzilla.samba.org/show_bug.cgi?id=11413 makes MPD
crash after at most a minute of using the plugin.  Since this bug is
five years old already and it doesn't look like it will ever be fixed,
all libsmbclient code in MPD is scheduled for removal.  For now, the
plugin is disabled by default so people are less likely to hit the
crash bug.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/991

doc/plugins.rst

@@ -71,6 +71,11 @@ Load music files from a SMB/CIFS server. It is used when
 :code:`music_directory` contains a ``smb://`` URI, for example
 :samp:`smb://myfileserver/Music`.
 
+Note that :file:`libsmbclient` has a serious bug which causes MPD to
+crash, and therefore this plugin is disabled by default and should not
+be used until the bug is fixed:
+https://bugzilla.samba.org/show_bug.cgi?id=11413
+
 nfs
 ---
 

doc/user.rst

@@ -86,7 +86,7 @@ For example, the following installs a fairly complete list of build dependencies
       libpulse-dev libshout3-dev \
       libsndio-dev \
       libmpdclient-dev \
-      libnfs-dev libsmbclient-dev \
+      libnfs-dev \
       libupnp-dev \
       libavahi-client-dev \
       libsqlite3-dev \