Find a better metavariable syntax in documentation

[osa1]

In the regex section we use <regex> syntax for metavariable "regex", but that syntax does not look great in the next section when we show Rust code. For example, we say things like:

If lexer is declared as Lexer -> Token;
...
LexerAction<Token>

Ideally we should indicate that Lexer in LexerAction is the user-specified lexer name, and Token is the return type (RHS of the arrow).

Some options:

• Keep using <...>: <Lexer>Action<<Token>>. I think double angle brackets around Token don't look good.
• Use $(...): $(Lexer)Action<$(Token)>. I like this better, because it looks somewhat similar to Rust macro syntax for splicing. (though you can't generate new symbols using this syntax in Rust, I think).
• Other ideas?