Improve docs for `context` param

[shirgoldbird]

Many users are confused by our context param, and expect to prompt it like an LLM. That strategy won't work (but it will work with custom_instructions.

The context parameter is:

• Optimized for document flow context
  • Works best with before/after paragraph information
  • Example: translating paragraph 3 benefits from paragraphs 1-2 and 4-5 as context
  • Less predictable for general contextual information (tone, campaign details, cultural context)
• Technical implementation feeds additional information during translation process
  • Model sees surrounding text content
  • Effect may vary by document type and structure
• Can be used for e.g. acronym consistency
  • First mention might be expanded, subsequent uses stay consistent based on context