Better document our support for numeric literals (and literals/defaults in general?)

[mhammond]

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Via #2543, our docs for literals in proc-macros aren't great - "String, integer, float, and boolean literals".

Also, via UDL, numeric literals carry a radix etc - so we've always supported rendering literals as hex or decimal etc in the bindings. Not sure how much of this is supported in proc-macros, but we should find out.

Ultimately it's probably better still for an entire page dedicated to defaults and literals, but small steps to get started.