Fix docs of "contextual abstractions"

[ShapelessCat]

• given clause --> using clause
• implicit function type --> context function type
• Fix typos
• Change one unordered list to ordered list
• Fix two code examples
• Separate code blocks from text by surrounding them with white lines -- follow the markdown convention to improve readability
• Remove unnecessary indentations in code blocks -- some code blocks indent the outmost level, some code blocks don't.

[dottybot]

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