cmd/doc: implement "go doc struct.field"

cmd/doc: implement "go doc struct.field"

By analogy with the handling of methods on types, show the documentation
for a single field of a struct.

	% go doc ast.structtype.fields
	struct StructType {
	    Fields *FieldList  // list of field declarations
	}
	%

Fixes #19169.

Change-Id: I002f992e4aa64bee667e2e4bccc7082486149842
Reviewed-on: https://go-review.googlesource.com/38438
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>