net/http/pprof: link docs to runtime/pprof

net/http/pprof: link docs to runtime/pprof

And add some documentation for the debug query param.

Fixes #27737
Fixes #53971

Change-Id: I629aaa2d4a43175381eb04872f1caad238519a41
Reviewed-on: https://go-review.googlesource.com/c/go/+/421635
Run-TryBot: Cherry Mui <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>