Fix oxidation document

[shyamd]

Fixes some minor bugs with the oxidation state document and adds proper descriptions
Also fixes MPID to properly validate integers

[codecov-commenter]

Codecov Report

Merging #201 (21df762) into main (b738071) will increase coverage by 0.17%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #201      +/-   ##
==========================================
+ Coverage   94.84%   95.01%   +0.17%     
==========================================
  Files          26       26              
  Lines        1823     1825       +2     
==========================================
+ Hits         1729     1734       +5     
+ Misses         94       91       -3     

Impacted Files	Coverage Δ
emmet-core/emmet/core/mpid.py	91.48% <100.00%> (+4.82%)	⬆️
emmet-core/emmet/core/oxidation_states.py	73.17% <100.00%> (+2.43%)	⬆️

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