TraceProcessor: add docs/help for RPC mode (--httpd/-D)
TraceProcessor: add docs/help for RPC mode (--httpd/-D)

When passing --httpd / -D to trace_processor_shell:
1. Print some minimal instructions on stderr.
2. If the user mistakenly opens http://127.0.0.1:9001 print a
   meaningful page with doc links.

Bug: 193627171
Change-Id: I92fc9d2914cef93b4ac1e35b3cb720e3665917ca
google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto