Document how tokens are compared in declarative macros

I wondered whether something like this would compile:

macro_rules! foo {
    ( r###"hello"### ) => {}; // 3 #-es
}

foo!( r##"hello"## ); // 2 #-es

It appears it does not, but this is not documented on the reference chapter about Macros By Example. There should be a section explaining the exact rules by which tokens are compared; there may be existing code out there that relies on behavior like this. Other examples I can think of:

• Doc comment tokens in matchers are currently ignored.
• Raw idents do not match non-raw idents (and vice versa).
• Suffixed literals do not match unsuffixed literals (and vice versa).

Start with the Rust Reference chapter on Macros By Example and inspect the linked rustc_expand/src/mbe/macro_parser.rs location. Trace the documented and implementation behavior for raw strings, doc comments, raw identifiers, and suffixed literals, then add a section explaining token comparison with examples. Done means the relevant matching rules and the reported raw-string case are clearly covered.