Better document existing utility methods

[Serial-ATA]

Having looked through multiple projects over the years, it's clear that Lofty's API either isn't the best documented or the easiest to understand. Some patterns keep showing up, namely:

• Not using {Tag::take_strings, Tag::get_strings}
  • Many projects could use take_strings instead of repeatedly cloning, since most don't write tags back to files.
  • Additionally, many projects could benefit from not having to iterate TagItems and check the ItemValue. get_strings exists for that purpose.
  • Finally, I have seen some projects that don't know about ItemKey. That makes it appear as though Lofty is only capable of providing the items accessible through Accessor. (!!!)
• Not using Tag::get_picture_type
  • Many will iterate all pictures to find the one they want. That isn't necessary.

I consider this an issue with the docs, not with the projects. I believe there are other things I have found in the past, but this is what I could think of for now.