MigrateSkipRowException should document that it is for use within migration process plugins

Problem/Motivation

Looking at MigrateSkipRowException, you might think you can throw it anywhere in a migration.

However, for instance, throwing it in a destination plugin causes an error. It looks like it's caught in:

- processing a row
- invoking migrate_prepare_row() hooks
- MigrationLookup -- which might be because that involves processing a row?

Steps to reproduce

Proposed resolution

Add documentation to the exception class docblock.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet