Trie API documentation updates

[scorbajio]

This PR addresses issues mentioned in #3265.

[codecov]

Codecov Report

Attention: 12 lines in your changes are missing coverage. Please review.

Comparison is base (0ea97e7) 88.35% compared to head (e9f7ae1) 86.89%.

Additional details and impacted files

Flag	Coverage Δ
block	88.57% <ø> (?)
blockchain	91.60% <ø> (?)
client	84.65% <ø> (?)
common	98.25% <ø> (?)
devp2p	82.12% <ø> (?)
ethash	∅ <ø> (?)
evm	74.54% <ø> (?)
genesis	99.98% <ø> (?)
statemanager	75.86% <ø> (?)
trie	89.12% <94.54%> (?)
tx	95.89% <ø> (?)
util	89.13% <ø> (?)
vm	80.16% <ø> (?)
wallet	88.35% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

[holgerd77]

LGTM