Docstrings improvements for NameID and argtree.add_path

[sheilatron]

Making docstrings more explanatory for NameID and argtree.add_path

I ran across these while stepping through tests, and writing docstrings
helps me understand them. Please let me know if you agree/disagree
with the added content. Thanks!

Sphinx docs build output said:

pysaml2/src/saml2/argtree.py:docstring of saml2.argtree.add_path:15: WARNING: Unexpected indentation.

I don't know if that matters, since I did not see any content generated out of docstrings using Sphinx autoclass or autofunction directives.

All Submissions:

• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you added an explanation of what problem you are trying to solve with this PR?
• Have you added information on what your changes do and why you chose this as your solution?
• Have you written new tests for your changes?
• Does your submission pass tests?
• This project follows PEP8 style guide. Have you run your code against the 'flake8' linter?

[codecov-io]

Codecov Report

Merging #538 into master will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #538      +/-   ##
==========================================
- Coverage   65.52%   65.48%   -0.04%     
==========================================
  Files         103      103              
  Lines       25660    25660              
==========================================
- Hits        16814    16804      -10     
- Misses       8846     8856      +10

Impacted Files	Coverage Δ
src/saml2/argtree.py	88.52% <ø> (ø)	⬆️
src/saml2/saml.py	92.28% <ø> (ø)	⬆️
src/saml2/validate.py	74.11% <0%> (-4.71%)	⬇️
src/saml2/__init__.py	86.52% <0%> (+0.18%)	⬆️
src/saml2/time_util.py	87.42% <0%> (+0.59%)	⬆️

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[sheilatron]

Something seems off with the code coverage job. It's showing code changes which don't belong to this PR, or at least are not included in the two commits.

[c00kiemon5ter]

Thanks!

Regarding the coverage report, I am not sure what is wrong, but I can see what you're saying. I will look into it as time allows.