Change multiline filter to multiline codec

[dedemorton]

Changes for elastic/logstash#6342

Removes mention of multiline filter plugin from the source for the plugin docs.

[acchen97]

@jordansissel minor doc change here, just want to confirm multiline codec works well with pipe. I'm not sure of all the input plugins that work with line vs bytes.

[jordansissel]

@acchen97 confirmed multiline codec seems to work with the pipe input:

% bin/logstash -e 'input { pipe { command => "iostat -m 1 3" codec => multiline { pattern => "^avg-cpu" negate => true what => previous } } } output { stdout { } }'

2017-01-02T23:55:10.432Z localhost.localdomain avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           0.37    0.01    0.05    0.03    0.00   99.55
Device:            tps    MB_read/s    MB_wrtn/s    MB_read    MB_wrtn
sda               0.19         0.00         0.00       1563       2853
dm-0              0.28         0.00         0.00       1555       2853
2017-01-02T23:55:11.431Z localhost.localdomain avg-cpu:  %user   %nice %system %iowait  %steal   %idle
          41.41    0.00    2.02    1.52    0.00   55.05
Device:            tps    MB_read/s    MB_wrtn/s    MB_read    MB_wrtn
sda               5.00         0.00         0.02          0          0
dm-0              4.00         0.00         0.02          0          0
2017-01-02T23:55:21.440Z localhost.localdomain avg-cpu:  %user   %nice %system %iowait  %steal   %idle
          15.66    0.00    1.52    1.52    0.00   81.31
Device:            tps    MB_read/s    MB_wrtn/s    MB_read    MB_wrtn
sda               3.00         0.00         0.01          0          0
dm-0              3.00         0.00         0.01          0          0