parser_json: Document new stream_buffer_size param

See fluent/fluentd#2381

In addition to documenting the parameter, update the `in_exec` and
`out_exec_filter` articles to provide a workaround for users
encountering this problem.
This note exists in the v0.12 version of the `in_exec` page but was
absent in the v1.0 docs, so add the whole note.

Closes #624

Signed-off-by: Ben Wheatley <[email protected]>

Author: benwh

docs/v1.0/in_exec.txt

@@ -25,6 +25,8 @@ You can run a program periodically or permanently. To run periodically, please u
 
 NOTE: Please see the <a href="config-file">Config File</a> article for the basic structure and syntax of the configuration file.
 
+NOTE: When using the json format in &lt;parse&gt; section, this plugin uses the Yajl library to parse the program output. Yajl buffers data internally so the output isn't always instantaneous. See the LINK:[parser_json](parser_json#stream_buffer_size) documentation to adjust this.
+
 ## Plugin helpers
 
 * [compat_parameters](api-plugin-helper-compat_parameters)

docs/v1.0/out_exec_filter.txt

@@ -27,7 +27,7 @@ The `out_exec_filter` Buffered Output plugin (1) executes an external program us
 
 NOTE: Please see the LINK:[Config File](config-file) article for the basic structure and syntax of the configuration file.
 
-NOTE: When using the json format in <parse> section, this plugin uses the Yajl library to parse the program output. Yajl buffers data internally so the output isn't always instantaneous.
+NOTE: When using the json format in <parse> section, this plugin uses the Yajl library to parse the program output. Yajl buffers data internally so the output isn't always instantaneous. See the LINK:[parser_json](parser_json#stream_buffer_size) documentation to adjust this.
 
 ## Supported modes
 

docs/v1.0/parser_json.txt

@@ -14,6 +14,17 @@ See [Parse section configurations](parse-section)
 
 Set JSON parser.
 
+### stream_buffer_size
+
+| type    | default | version |
+|:-------:|:-------:|:-------:|
+| integer | 8192    | 1.4.3  |
+
+The amount of data, in bytes, that the Yajl parser will wait for before parsing and emitting JSON objects.
+See the [Yajl documentation](https://www.rubydoc.info/github/brianmario/yajl-ruby/Yajl%2FParser:parse) for further details.
+
+This will only take effect when the parser is being used to process an IO stream, e.g. in the context of the `out_exec_filter` or `in_exec` plugins.
+
 ## Example
 
     :::text