document what unsafety means

Closes #9144
auto merge of #9258 : thestinger/rust/doc, r=catamorphism

Closes #9144
Added support for a `\0` escape sequence.

This commit adds support for `\0` escapes in character and string literals.

Since `\0` is equivalent to `\x00`, this is a direct translation to the latter
escape sequence. Future builds will be able to compile using `\0` directly.

Also updated the grammar specification and added a test for NUL characters.