cmd/go: behaviour of "go doc" in module mode is not documented

[dolmen]

The documentation for go doc still refers to GOPATH mode. No mention of Go module mode (which is now the default behavior).

What version of Go are you using (go version)?

$ go version
master

Does this issue reproduce with the latest release?

Yes (1.21rc2).
See https://pkg.go.dev/cmd/go@master#hdr-Show_documentation_for_package_or_symbol

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
1.21rc2

What did you do?

$ go help doc

Or online: https://pkg.go.dev/cmd/go@master#hdr-Show_documentation_for_package_or_symbol

What did you expect to see?

Documentation of the "go doc" behavior in Go module mode, with no mention of GOPATH.

What did you see instead?

When run with one argument, the argument is treated as a Go-syntax-like
representation of the item to be documented. What the argument selects depends
on what is installed in GOROOT and GOPATH [...]

[cagedmantis]

/cc @robpike @mvdan

[dolmen]

Still applies to Go 1.25.

[dolmen]

Also, the Go Module Reference doesn't list go mod as being module-aware in the list of "Builds commands" section while it is also a command that loads information about packages.

[dmitshur]

CC @golang/command-line.