replace maybe locate section with a context section

[SillyFreak]

Content:

• remove the obsolete maybe-locate section
• describe the main points of the context feature: giving access to location-specific information in the document, and opaqueness
• give a rule of thumb on how to use context as library authors: require the user to provide context if the return value may need to be processed further
• discuss limitations of the rule of thumb

[tingerrr]

Thanks!