Add doc for Tag#name if no tag name is supplied

[ksss]

Description

Added documentation for YARD::Tags::Tag#name, which could be nil.

The following is a typical example.

YARD::DocstringParser.new.parse("@return [void]").tags
#=> [#<YARD::Tags::Tag:0x0000000105294108 @name=nil, @tag_name="return", @text="", @types=["void"]>]

Completed Tasks

• I have read the Contributing Guide.
• The pull request is complete (implemented / written).
• Git commits have been cleaned up (squash WIP / revert commits).
• I wrote tests and ran bundle exec rake locally (if code is attached to PR).