increase default buffer size to 4 MB

2 MB was too small for cross-fading a 24 bit file.  Increasing to 4 MB
is still not too large for weak machines, but is enough for
cross-fading.

doc/mpd.conf.5

@@ -195,7 +195,7 @@ This is the gain (in dB) applied to songs with ReplayGain tags.
 If yes, mpd will normalize the volume of songs as they play.  The default is no.
 .TP
 .B audio_buffer_size <size in KiB>
-This specifies the size of the audio buffer in kibibytes.  The default is 2048,
+This specifies the size of the audio buffer in kibibytes.  The default is 4096,
 large enough for nearly 12 seconds of CD-quality audio.
 .TP
 .B buffer_before_play <0-100%>

doc/mpdconf.example

@@ -382,7 +382,7 @@ input {
 # this may have undesired effects. Don't change this if you don't know what you
 # are doing.
 #
-#audio_buffer_size		"2048"
+#audio_buffer_size		"4096"
 #
 # This setting controls the percentage of the buffer which is filled before 
 # beginning to play. Increasing this reduces the chance of audio file skipping,