Some minor lints

[nyurik]

• use #[must_use] for Self-returning fn
• some minor docs for panics
• use Self instead of a specific type in some cases

I used this command to test:

❯ cargo clippy -- -W clippy::default_trait_access -W clippy::missing_panics_doc -W clippy::return-self-not-must-use -W clippy::semicolon-if-nothing-returned -W clippy::trait-duplication-in-bounds

There is only one issue remaining that I'm not certain how to fix (or if its even needed to be fixed?):

warning: this trait bound is already specified in the where clause
  --> postgres/src/lib.rs:52:46
   |
52 |     <Tls as MakeTlsConnect<Socket>>::Stream: Send + Sync,
   |                                              ^^^^
   |
   = note: requested on the command line with `-W clippy::trait-duplication-in-bounds`
   = help: consider removing this trait bound
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#trait_duplication_in_bounds

See #131 . I tried to add a clippy.toml file to make this more consistent, but that seems not to work properly -- https://github.com/rust-lang/rust-clippy/issues/9265

[djc]

Looks to me like the trait-duplication-in-bounds issue is a clippy false positive, might be worth filing an issue about that.

[nyurik]

thx, fixed