Added documentation for avs_http
Added documentation for avs_http

Summary:
Fixes T1345

Partially copied from libCWMP

Test Plan: n/a

Reviewers: mkrawiec, marian

Reviewed By: mkrawiec, marian

Maniphest Tasks: T1345

Differential Revision: https://phabricator.avsystem.com/D5237

AVSystem/avs_commons,AVSystem/avs_commons,AVSystem/avs_commons,AVSystem/avs_commons