[AIRFLOW-2404] Add additional documentation for unqueued task

[AetherUnbound]

Make sure you have checked all steps below.

JIRA

• My PR addresses the following Airflow JIRA issues and references them in the PR title. For example, "[AIRFLOW-XXX] My Airflow PR"
  • https://issues.apache.org/jira/browse/AIRFLOW-2404

Description

• Here are some details about my PR, including screenshots of any UI changes:
  • I've added some additional explanation as to why a task might not have started. This came about as we were trying to understand why our scheduler wasn't queueing more DAG runs
    

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:
  • This is a documentation change and does not need unit tests

Commits

• My commits all reference JIRA issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Documentation

• In case of new functionality, my PR adds documentation that describes how to use it.

Code Quality

• Passes git diff upstream/master -u -- "*.py" | flake8 --diff

[Fokko]

Thanks @AetherUnbound Improvements on documentation are highly appreciated.

[codecov-io]

Codecov Report

Merging #3286 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #3286   +/-   ##
======================================
  Coverage    75.9%   75.9%           
======================================
  Files         197     197           
  Lines       14702   14702           
======================================
  Hits        11159   11159           
  Misses       3543    3543

Impacted Files	Coverage Δ
airflow/www/views.py	71.41% <ø> (ø)	⬆️

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[AetherUnbound]

No worries! This should be ready @Fokko