Added API HTTP Return Code Documentation

[caronc]

Description:

Related issue (if applicable): refs #158

Improved documentation describing different HTTP Return codes when using the Apprise API

Checklist

• [x] The code change is tested and works locally.
• [x] There is no commented out code in this PR.
• [x] No lint errors (use flake8)
• [x] Tests added

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (19a48a6) 94.87% compared to head (5ba7981) 94.87%.

:exclamation: Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #160 +/- ## ======================================= Coverage 94.87% 94.87% ======================================= Files 7 7 Lines 702 702 ======================================= Hits 666 666 Misses 36 36 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.