nvim: fix and add documentation for APIs

[zchee]

Fix and add documentation for APIs.

[codecov]

Codecov Report

Merging #82 (5df923d) into master (f801acd) will increase coverage by 2.5%.
The diff coverage is 43.2%.

@@           Coverage Diff            @@
##           master     #82     +/-   ##
========================================
+ Coverage    58.0%   60.6%   +2.5%     
========================================
  Files          11      12      +1     
  Lines        2140    2079     -61     
========================================
+ Hits         1243    1260     +17     
+ Misses        763     684     -79     
- Partials      134     135      +1     

Flag	Coverage Δ
linux-1.13	60.6% <43.2%> (+2.5%)	⬆️
linux-1.14	60.6% <43.2%> (+2.5%)	⬆️
linux-1.15	60.6% <43.2%> (+2.5%)	⬆️
macos-1.13	60.6% <43.2%> (+2.5%)	⬆️
macos-1.14	60.6% <43.2%> (+2.5%)	⬆️
macos-1.15	60.6% <43.2%> (+2.5%)	⬆️

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Impacted Files	Coverage Δ
msgpack/decode.go	64.3% <ø> (ø)
msgpack/rpc/endpoint.go	58.0% <ø> (ø)
msgpack/unpack.go	86.0% <ø> (ø)
nvim/helpers.go	82.6% <ø> (+5.6%)	⬆️
nvim/nvim.go	70.4% <ø> (+1.6%)	⬆️
nvim/plugin/plugin.go	31.5% <ø> (ø)
nvim/types.go	0.0% <0.0%> (ø)
nvim/api.go	35.0% <43.5%> (ø)
... and 1 more

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