x/tools/cmd/stringer: document option 'trimprefix'

[mtenberge]

Go version

Online 'Latest' version on the pkg.go.dev documentation

Output of go env in your module/workspace:

irrelevant

What did you do?

Viewing the online documentation on stringer at https://pkg.go.dev/golang.org/x/tools/cmd/stringer.

What did you see happen?

Nowhere in the package documentation is any mention of the stringer command line option trimprefix.

What did you expect to see?

The Usage() output does mention trimprefix, see https://cs.opensource.google/go/x/tools/+/refs/tags/v0.37.0:cmd/stringer/stringer.go;drc=6016188d8e58282ea0a17064039059f275da5d01;l=96

I'd expect the option to also be mentioned in the package comments/documentation.

[mknyszek]

CC @golang/tools-team

[gopherbot]

Change https://go.dev/cl/706455 mentions this issue: cmd/stringer: document option -trimprefix