frame/proc-macro: Refactor code for better readability (#4712)

Small refactoring PR to improve the readability of the proc macros.
- small improvement in docs
- use new `let Some(..) else` expression
- removed extra indentations by early returns

Discovered during metadata v16 poc, extracted from:
https://github.com/paritytech/polkadot-sdk/pull/4358

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Signed-off-by: Alexandru Vasile <[email protected]>
Co-authored-by: Bastian Köcher <[email protected]>
Co-authored-by: command-bot <>
Co-authored-by: gupnik <[email protected]>