parser_json: Document new stream_buffer_size param

[benwh]

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]