[DOC] doc fix for vocab.subwords

[liusy182]

Description

This PR fixes a few documentation inconsistencies in package gluonnlp.vovab

Checklist

Essentials

• PR's title starts with a category (e.g. [BUGFIX], [MODEL], [TUTORIAL], [FEATURE], [DOC], etc)
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage
• Code is well-documented

Changes

• use the same parameter name in method signature and method documentation.

• explicitly document RuntimeException

Comments

[codecov]

Codecov Report

❗ No coverage uploaded for pull request head (vocab-doc@6d08590). Click here to learn what that means.
The diff coverage is n/a.

[codecov]

Codecov Report

Merging #885 into master will decrease coverage by 0.04%.
The diff coverage is 75%.

@@            Coverage Diff             @@
##           master     #885      +/-   ##
==========================================
- Coverage   90.43%   90.38%   -0.05%     
==========================================
  Files          66       66              
  Lines        6367     6367              
==========================================
- Hits         5758     5755       -3     
- Misses        609      612       +3

Impacted Files	Coverage Δ
src/gluonnlp/vocab/subwords.py	81.13% <75%> (ø)	⬆️
src/gluonnlp/data/utils.py	70.74% <0%> (-2.05%)	⬇️

[mli]

Job PR-885/1 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-885/1/index.html

[leezu]

Thanks!