Perf tuning documents

[yangchi]

There has been various reports in mvfst and proxygen repo regarding performance problem, for example: facebook/proxygen#294, and a few outsider blogs and other reports on it. A few of them is simply due to the default parameters come with the code. I think we need to either change them, or provide documentation to help people get some of the most basic things right, and raise the awareness of known problem (for example: we use a fixed and small MSS)