Update Natural Language docs and guides

[quartzmo]

Update docs using new content from https://cloud.google.com/natural-language/docs/, as soon as it is available.

• Update language limitations to full support for English, Spanish, and Japanese
• Update all Syntax examples to use new syntax method. (Syntax: Use analyze_syntax where possible #1037)
• Resolve all docs TODOs (Most or all will be found in PartOfSpeech. (Syntax: Add new attributes to part_of_speech #1038)
• Improve descriptions of EntityMention.type values. (Entities: Add EntityMention.type attribute #1039)
• Improve description of mid attribute. (Entities: Add Entity#mid #1040)
• Improve description of score attribute. (Sentiment: Add Sentence#sentiment #1041, Sentiment: Replace polarity with score #1042 )
• Update all headline Sentiment examples to show the new sentiment methods on Sentence. (Sentiment: Add sentences, with Sentiment at sentence level #1043)
• Add example, description, and possible values for encoding_type param. (Sentiment: Add encoding_type param to Project#sentiment, Document#sentiment #1044)