_docs: update the manual

[quasilyte]

Refs #149

[sebastien-rosset]

It may be worth having an example where the interface name requires an import. Presumably the import does not (and cannot) be done in this function; it needs to be done in the rule right before m.Match, for every rule that uses implementsStringer

[quasilyte]

The GetInterface() (and GetType()) methods accept a FQN. So it can be a github.com/a/b.Type without Import. I also made Implements in DSL accept FQN; so Import is mostly needed for the brevity now.

[quasilyte]

It accepts a typeName parameter:

go-ruleguard/dsl/dsl.go

Lines 166 to 169 in c73ee1c

	// A type name can be:
	// - builtin type name: `error`, `string`, etc.
	// - qualified name from a standard library: `io.Reader`, etc.
	// - fully-qualified type name, like `github.com/username/pkgname.TypeName`