Improve teamcity agent setup documentation and service script

Summary:
These minor improvements comes from my experience in setting up an agent
from scratch using this documentation.

Test Plan:

Read the doc and setup an agent

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2751