Improve doc for pallet macro and config macro (#4146)

Improve doc:

* the pallet macro is actually referring to 2 places, for the module and
for the struct placeholder but doesn't really clarify it (I should have
named the latter just `pallet_struct` or something but it is a bit late)

* The doc of `with_default` is a bit confusing too IMO.

CC @Kianenigma



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Co-authored-by: Liam Aharon <[email protected]>