Add docs for HTTP/2 stats

[masaori335]

We have some stats for HTTP/2, but there're no docs.

$ traffic_ctl metric match http2
proxy.process.http2.total_client_streams 24004
proxy.process.http2.total_transactions_time 62595216907000
proxy.process.http2.total_client_connections 217
proxy.process.http2.connection_errors 0
proxy.process.http2.stream_errors 0
proxy.process.http2.session_die_default 202
proxy.process.http2.session_die_other 0
proxy.process.http2.session_die_eos 15
proxy.process.http2.session_die_active 0
proxy.process.http2.session_die_inactive 0
proxy.process.http2.session_die_error 0
proxy.process.http2.current_client_sessions 0
proxy.process.http2.current_client_streams 0

[masaori335]

It should be added under Core Statistics page.

[bryancall]

@masaori335 Can you please make sure that the docs are added before the 9.0.0 branching

[masaori335]

I'll add doc soon.