Fix build when #![deny(missing_docs)] is in force

[asomers]

Also, add mor examples to mockall_examples

[asomers]

@codehearts everything should build fine with #[deny(missing_docs)] now. Could you please check it? I tried to add useful docs everywhere. But there were a few modules that aren't intended for direct user interaction, yet can't be hidden because they contain things that are intended for user interaction. For those I simply allowed missing docs.

[asomers]

The build failure is due to a stricter check for uninitialized data in the latest nightly compiler. I'll fix it in a separate PR.

[codehearts]

This resolved it for me, thank you so much for making time for this!

[asomers]

Thanks for reporting it. And happy hacking!