Fix and document extracting help form docstring

[tiangolo]

🐛 📝 Fix and document extracting help form docstring.

This should fix/related to #3

[codecov-io]

Codecov Report

Merging #10 into master will decrease coverage by 2.32%.
The diff coverage is 51.19%.

@@            Coverage Diff             @@
##           master      #10      +/-   ##
==========================================
- Coverage   75.27%   72.94%   -2.33%     
==========================================
  Files          16       17       +1     
  Lines         554      621      +67     
==========================================
+ Hits          417      453      +36     
- Misses        137      168      +31

Impacted Files	Coverage Δ
...test_tutorial/test_first_steps/test_tutorial003.py	100% <ø> (ø)	⬆️
...test_tutorial/test_first_steps/test_tutorial001.py	100% <ø> (ø)	⬆️
...test_tutorial/test_first_steps/test_tutorial004.py	100% <ø> (ø)	⬆️
...test_tutorial/test_first_steps/test_tutorial002.py	100% <ø> (ø)	⬆️
...test_tutorial/test_first_steps/test_tutorial005.py	100% <ø> (ø)	⬆️
...test_tutorial/test_first_steps/test_tutorial006.py	100% <100%> (ø)
docs/src/first_steps/tutorial006.py	100% <100%> (ø)
typer/completion.py	70% <33.33%> (+1.57%)	⬆️
typer/main.py	48.87% <9.3%> (-4.75%)	⬇️
typer/__init__.py
... and 2 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a650881...f00e100. Read the comment docs.