Add Time_System_Timezone parser option docs (fluent#1388)

* Add Time_System_Timezone parser option docs

* Mention TZ environment variable in the Time_System_Timezone docs

pipeline/parsers/configuring-parser.md

@@ -32,6 +32,7 @@ Multiple parsers can be defined and each section has it own properties. The foll
 | Time\_Format | Specify the format of the time field so it can be recognized and analyzed properly. Fluent-bit uses `strptime(3)` to parse time so you can refer to [strptime documentation](https://linux.die.net/man/3/strptime) for available modifiers. |
 | Time\_Offset | Specify a fixed UTC time offset \(e.g. -0600, +0200, etc.\) for local dates. |
 | Time\_Keep | By default when a time key is recognized and parsed, the parser will drop the original time field. Enabling this option will make the parser to keep the original time field and it value in the log entry. |
+| Time\_System\_Timezone | If there is no timezone (`%z`) specified in the given `Time_Format`, enabling this option will make the parser detect and use the system's configured timezone. The configured timezone is detected from the [`TZ` environment variable](https://www.gnu.org/software/libc/manual/html_node/TZ-Variable.html). |
 | Types | Specify the data type of parsed field. The syntax is `types <field_name_1>:<type_name_1> <field_name_2>:<type_name_2> ...`. The supported types are `string`\(default\), `integer`, `bool`, `float`, `hex`. The option is supported by `ltsv`, `logfmt` and `regex`. |
 | Decode\_Field | Decode a field value, the only decoder available is `json`. The syntax is: `Decode_Field json <field_name>`. |
 | Skip\_Empty\_Values | Specify a boolean which determines if the parser should skip empty values. The default is `true`. |