docs: missing NodePort service type description comment

[chauhanshubham]

What this PR does / why we need it:
Adds missing comment for NodePort type service

[chauhanshubham]

cc: @zirain wrt #1392 (comment)
the function itself is a service type checker, so I think it's fine to leave it as is?
While checking that out I found that I had missed a comment around the NodePort service type, so added that in this PR.

[codecov]

Codecov Report

Merging #1401 (59da5b8) into main (297fcf1) will decrease coverage by 0.09%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1401      +/-   ##
==========================================
- Coverage   62.44%   62.36%   -0.09%     
==========================================
  Files          79       79              
  Lines       11084    11084              
==========================================
- Hits         6921     6912       -9     
- Misses       3707     3714       +7     
- Partials      456      458       +2     

see 2 files with indirect coverage changes

[zirain]

cc: @zirain wrt #1392 (comment) the function itself is a service type checker, so I think it's fine to leave it as is? While checking that out I found that I had missed a comment around the NodePort service type, so added that in this PR.

it's ok now, leave it for someone who is new to this project?

[qicz]

nit: pr title ref https://gateway.envoyproxy.io/latest/dev/CONTRIBUTING.html#submitting-a-pr