Missing documentation about concurrent runs of MigrationHarness::run_pending_migrations

[ISibboI]

Setup

Versions

• Diesel: 2.1.0

Problem Description

The documentation of MigrationHarness::run_pending_migrations does not mention if the method can safely be called concurrently from e.g. two different processes.

Will this lead to the same migrations being applied twice, will one of the method calls fail (with a panic or an error?), or will all calls simply block until the migrations are finished?

What are you trying to accomplish?

Automatically apply pending migrations on application startup, but the application is architectured to have multiple backend processes running concurrently.

Checklist

• I have already looked over the issue tracker and the discussion forum for similar possible closed issues.

• This issue can be reproduced on Rust's stable channel. (Your issue will be
  closed if this is not the case)
• This issue can be reproduced without requiring a third party crate

[weiznich]

I'm happy to accept a PR that adds this information to the documentation.