Documentation comments from Reuben

[mborland]

Minor comments on documentation:

• The first example uses to_chars(... sizeof(buffer) - 1), which seems
  to imply a NULL-terminator somewhere. Since there is none, I think it
  should just be sizeof(buffer).
• Docs don't specify the behavior about NULL-terminators. Either
  specify "adheres to standard" or clarify that no NULL-terminator is
  written.
• IMO the front page should clearly state that this library is
  compliant with the charconv standard header, noting any exceptions.
• from_chars overview is missing semicolons in the friends section.
• Please use monospace in the discussion to refer to identifiers, like
  [first, last).
• Basic usage: from_chars name is qualified but from_chars_result is not.
• Code snippets wrap, which makes them difficult to read.

cc: @anarthal