Polish `missing_doc_code_examples` tests and the lint itself

[GuillaumeGomez]

@lolbinarycat made a very detailed list of what remains to be done for this lint to be complete here. I will not mark this issue as E-Easy or as mentored because some points here will require discussions with the team.

From those mentioned in the above PR (checked=tests exist):

• Impl blocks
• Enum variants
• Struct/union fields
• (associated) Type aliases
• Statics/constants
• Modules
• Inlined reexports (cross-crate)
• Inlined reexports (intra-crate)
• Non-inlined reexports (cross-crate)
• Non-inlined reexports (intra-crate)
• Inherient associated constants
• Trait associated constants
• Required trait associated types
• Optional trait associated types

Additional tests needed:

• All 3 kinds of macros (v2, macro_rules, proc_macro). Currently these (at least macro_rules macros) are not linted on, but there's a solid argument they should be.
• Trait definitions
• Trait items in a trait definition (all of them, including both required and non-required methods)
• pub extern crate items.
• Everything that can appear in an extern block (functions, statics, and types)

In addition to all this: private/hidden items are not checked (also clarify if that behavior is changed by --document-private-items and/or --document-hidden-items).

Additionally, we should clarify what counts as a "code example". Does it have to be a doctest (i.e. does ignore not count)? Any rust code block? Any code block? Once we decide on a behavior we want, we'll also need tests for that behavior.

[lolbinarycat]

I'm gonna take a pass at making this work better with macros.