Add info section about UDP buffer size limits

[joschi]

Network-based inputs have configuration options for setting the receive buffer size which is dependent on the system limits.

We should add an informational section to the documentation which explains how to set those limits, similar to https://access.redhat.com/documentation/en-US/JBoss_Enterprise_Web_Platform/5/html/Administration_And_Configuration_Guide/jgroups-perf-udpbuffer.html

[joschi]

Additional documentation sources:

• http://manpages.ubuntu.com/manpages/trusty/man7/tcp.7.html
• http://www.cyberciti.biz/faq/linux-tcp-tuning/
• https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/7/html/Performance_Tuning_Guide/sect-Red_Hat_Enterprise_Linux-Performance_Tuning_Guide-Networking-Configuration_tools.html#sect-Red_Hat_Enterprise_Linux-Performance_Tuning_Guide-Configuration_tools-Configuring_socket_receive_queues