runtime/pprof: document use of pprof package

Currently the pprof package gives almost no guidance for how to use it
and, despite the standard boilerplate used to create CPU and memory
profiles, this boilerplate appears nowhere in the pprof documentation.

Update the pprof package documentation to give the standard
boilerplate in a form people can copy, paste, and tweak. This
boilerplate is based on rsc's 2011 blog post on profiling Go programs
at https://blog.golang.org/profiling-go-programs, which is where I
always go when I need to copy-paste the boilerplate.

Change-Id: I74021e494ea4dcc6b56d6fb5e59829ad4bb7b0be
Reviewed-on: https://go-review.googlesource.com/25182


Reviewed-by: Rick Hudson <rlh@golang.org>