Document the real parsing, nesting, comment, escape and validation rules

[GrahamCampbell]

Several parser and loader behaviours that generate recurring questions were undocumented or described inaccurately, including how undefined and braceless nested variables resolve, that a hash begins a comment inside an unquoted value, which backslash escapes are valid inside double quotes, that byte order and getenv population differ from the tagline, and how immutability, the file options and parse-versus-load actually behave. This documents them, corrects the validation examples to match the real messages, and fixes a garbled sentence in the upgrading guide. This is stacked on #615.