x/tools/gopls: improve output for subcommands with no flags

[ainar-g]

Currently (golang/tools@6375436), the output of gopls check -h looks like this:

show diagnostic results for the specified file

Usage: check [flags] <filename>

Example: show the diagnostic results of this file:

  $ gopls check internal/lsp/cmd/check.go

	gopls check flags are:

That is, no flags are documented. If gopls check actually has flags, they should probably be documented, and if not, the output should probably not mention them at all.