net/http: PostForm docs should mention PATCH method

[mbertschler]

The net/http parses the request body for the PATCH HTTP method for quite some time now.
I think the docs should mention that PostForm is available not only for POST and PUT but also for PATCH, that users don't need to dig into the code to discover that.
gov1.4.1

[gopherbot]

CL https://golang.org/cl/9410 mentions this issue.