in_syslog: Add a description of source_address_key parameter (#1151)

Signed-off-by: Hiroshi Hatake <[email protected]>

Author: cosmo0920

pipeline/inputs/syslog.md

@@ -17,6 +17,7 @@ The plugin supports the following configuration parameters:
 | Buffer\_Chunk\_Size | By default the buffer to store the incoming Syslog messages, do not allocate the maximum memory allowed, instead it allocate memory when is required. The rounds of allocations are set by _Buffer\_Chunk\_Size_. If not set, _Buffer\_Chunk\_Size_ is equal to 32000 bytes \(32KB\). Read considerations below when using _udp_ or _unix\_udp_ mode. |  |
 | Buffer\_Max\_Size | Specify the maximum buffer size to receive a Syslog message. If not set, the default size will be the value of _Buffer\_Chunk\_Size_. |  |
 | Receive\_Buffer\_Size | Specify the maximum socket receive buffer size. If not set, the default value is OS-dependant, but generally too low to accept thousands of syslog messages per second without loss on _udp_ or _unix\_udp_ sockets. Note that on Linux the value is capped by `sysctl net.core.rmem_max`.| |
+| Source\_Address\_Key| Specify the key where the source address will be injected. | |
 
 ### Considerations