Fix clippy pedantic warnings on the async module

[cljoly]

Current issues:

warning: this method could have a `#[must_use]` attribute
  --> rusqlite_migration/src/asynch.rs:25:5
   |
25 |     pub fn new(ms: Vec<M<'static>>) -> Self {
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: add the attribute: `#[must_use] pub fn new(ms: Vec<M<'static>>) -> Self`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#must_use_candidate
   = note: `-W clippy::must-use-candidate` implied by `-W clippy::pedantic`

warning: docs for function returning `Result` missing `# Errors` section
  --> rusqlite_migration/src/asynch.rs:55:5
   |
55 |     pub async fn current_version(&self, async_conn: &AsyncConnection) -> Result<SchemaVersion> {
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#missing_errors_doc
   = note: `-W clippy::missing-errors-doc` implied by `-W clippy::pedantic`

warning: docs for function returning `Result` missing `# Errors` section
  --> rusqlite_migration/src/asynch.rs:82:5
   |
82 |     pub async fn to_latest(&self, async_conn: &mut AsyncConnection) -> Result<()> {
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#missing_errors_doc

warning: docs for function returning `Result` missing `# Errors` section
   --> rusqlite_migration/src/asynch.rs:116:5
    |
116 |     pub async fn to_version(&self, async_conn: &mut AsyncConnection, version: usize) -> Result<()> {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#missing_errors_doc

warning: docs for function returning `Result` missing `# Errors` section
   --> rusqlite_migration/src/asynch.rs:139:5
    |
139 |     pub async fn validate(&self) -> Result<()> {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#missing_errors_doc

warning: `rusqlite_migration` (lib) generated 5 warnings

Some can silenced maybe? Like the doc ones, when we merely link to the sync method?

[cljoly]

This is a follow-up of #29

[czocher]

Created a PR to fix this problem :). I found no way to show clippy that there's a related doc, but ignoring the warning is safe in this situation AFAIK.

[cljoly]

but ignoring the warning is safe in this situation AFAIK.

Yeah, looking at the PR it seems safe (the error case is documented in the blocking version).