runtime/pprof: document predefined profiles under Profiles

[rakyll]

It is backwards for pprof.Profile to document the predefined profiles rather than pprof.Profiles or pprof.Lookup. Given the documentation of the predefined is required by multiple APIs under the pprof package, we might consider documenting them under the package doc.

[gopherbot]

CL https://golang.org/cl/36326 mentions this issue.

[bradfitz]

The proposed CL was abandoned, so closing this.