[AIRFLOW-XXX] Add section on task lifecycle & correct casing in docs

[BasPH]

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-XXX
  • In case you are fixing a typo in the documentation you can prepend your commit with [AIRFLOW-XXX], code changes always need a Jira issue.

Description

• Here are some details about my PR, including screenshots of any UI changes:

• This PR corrects the casing for a few abbreviations and names in the docs: airflow -> Airflow & dag -> DAG.

• Added a section in the docs about task lifecycle. I often receive questions about it, and it helps to know what component (scheduler/worker) is responsible for what stage of a task.

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:

no tests needed

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 (not including Jira issue reference)
  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.
  • When adding new operators/hooks/sensors, the autoclass documentation generation needs to be added.
  • All the public functions and the classes in the PR contain docstrings that explain what it does

Code Quality

• Passes flake8

[codecov-io]

Codecov Report

Merging #4681 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4681      +/-   ##
==========================================
- Coverage   74.34%   74.33%   -0.01%     
==========================================
  Files         429      429              
  Lines       27950    27950              
==========================================
- Hits        20779    20778       -1     
- Misses       7171     7172       +1

Impacted Files	Coverage Δ
airflow/models/__init__.py	92.81% <0%> (-0.05%)	⬇️

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