chore: Update the Client file documentation

[vinifarias]

Short description of what this PR does:

• Adds and corrects the Client class methods documentation.

If you have questions, please send an email to Sendgrid, or file a Github Issue in this repository.

[SendGridDX]

All committers have signed the CLA.

[vinifarias]

Hi guys, I'm participating in hacktoberfest, so please mark this pull request with the hacktoberfest and difficulty: easy labels :)

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@12d28b7). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #111   +/-   ##
=========================================
  Coverage          ?   46.47%           
  Complexity        ?       36           
=========================================
  Files             ?        5           
  Lines             ?      213           
  Branches          ?       16           
=========================================
  Hits              ?       99           
  Misses            ?      110           
  Partials          ?        4

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/com/sendgrid/Client.java	30.28% <ø> (ø)	10 <0> (?)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 12d28b7...6e1befc. Read the comment docs.

[thinkingserious]

Hello @vinifarias,

Thanks again for the PR!

We want to show our appreciation by sending you some swag. Could you please fill out this form so we can send it to you? Thanks!

Team SendGrid DX