Macro docs use term "literal token" which is undefined and confusing

From https://doc.rust-lang.org/nightly/reference/macros-by-example.html#r-macro.decl.transcription.fragment:

When forwarding a matched fragment to another macro-by-example, matchers in the second macro will see an opaque AST of the fragment type. The second macro can’t use literal tokens to match the fragments in the matcher, only a fragment specifier of the same type. The ident, lifetime, and tt fragment types are an exception, and can be matched by literal tokens.

I don't know what a "literal token" is, but it's apparently not the same as a token of kind literal (i.e., $foo:literal), so the term is confusing and also not properly introduced.

Cc @petrochenkov

Start at the Rust Reference's macros-by-example transcription section linked in the issue and compare "literal token" with the literal fragment kind and the other fragment specifiers. Review the surrounding macro-matching terminology, then revise the explanation so the term is introduced and its distinction from literal fragments is clear.