Update docs for consistency

• add period to the end of the first line
• apply mix format for code in doc
• remove extra blank lines
• remove colon at the end of headings
• add lines around heading / list
• use :key not key
• use - over *
• remove unrendered alignment in markdown
• use @impl ModuleName and drop @doc false for callbacks
• refer callbacks
• place @moduledoc at the top of module
• use "argument", not "parameter"
• use headings from ##