Add code examples to the 'tags' package.

[sebright]

[sebright]

See #485 (review) for more suggestions on documentation for tags.

[sebright]

This issue is for adding examples to Javadocs in package-info.java and classes like TagContextBuilder or Tagger.

[songy23]

Duplicated of #1246.

[sebright]

This issue is about improving the Javadocs for the tagging, for example, TagContextBuilder and Tagger. I think the examples can be much simpler now that there aren't multiple key types, though.