no-piggyback-errors (#145)

Added "no-piggyback-errors" to parser flags.

Signed-off-by: Zsolt Gyulai (zgyulai) <zsolt.gyulai@quest.com>

Author: zsoltgyulai94

_includes/doc/admin-guide/options/source-flags.md

@@ -1,6 +1,6 @@
 ## flags()
 
-|Type:| assume-utf8, empty-lines, expect-hostname, kernel, no-hostname, no-multi-line, no-parse, sanitize-utf8, store-legacy-msghdr, store-raw-message, syslog-protocol, threaded, validate-utf8|
+|Accepted values:| `assume-utf8`, `empty-lines`, `expect-hostname`, `kernel`, `no-hostname`, `no-multi-line`, `no-parse`, `sanitize-utf8`, `store-legacy-msghdr`, `store-raw-message`, `syslog-protocol`, `threaded`, `validate-utf8`, `no-piggyback-errors`, `piggyback-errors`|
 |Default: |  empty set|
 
 *Description:* Specifies the log parsing options of the source.
@@ -139,5 +139,7 @@
     If the BOM[^1] character is missing, but the message is otherwise UTF-8
     compliant, {{ site.product.short_name }} automatically adds the BOM character to the
     message.
+- *no-piggyback-errors*: On failure, the original message will be left as it was before parsing, the value of `$MSGFORMAT` will be set to `syslog:error`, and a tag (see Tagging messages) will be placed on the message corresponding to the parser's failure.
+- *piggyback-errors*: On failure, the old behaviour is used (clearing the entire message then syslog-ng will generate a new message in place of the old one describing the parser's error).
 
 [^1]: The byte order mark (BOM) is a Unicode character used to signal the byte-order of the message text.