Added logging control (#667)

* * rocDecode: Added logging control
 - Message output from the core components is now controlled by the logging level, which can be set by an environment variable or other methods.

* * rocDecode/Logging control: Fixed a typo.

* * rocDecode/Logging control: Removed reference to the logger class from RocVideoDecoder utility, which results in build error on non-source install environment.

* * rocDecode/Logging control: Improved some wording in the docs.

docs/how-to/using-rocdecode.rst

@@ -218,6 +218,16 @@ Destroy the parser
 You need to call ``rocDecDestroyVideoParser()`` to destroy the parser object and free up all allocated
 resources at the end of video decoding.
 
+Logging control
+====================================================
+rocDecode core components can generate various logs during the decode session. The logs can be critical status reports, error reports, warnings, etc. 
+Each log has a significance level associated to it. The level can be from critical (0) to debug info related (4). Lower value has greater importance. 
+An internal logging level threshold can be set to control the verbosity of logging output from each rocDecode component. If a log's level value is less 
+than or equal to the logging level threshold, the log is output. Otherwise, the log is not output.
+
+The logging level threshold can be set by the environment variable ROCDEC_LOG_LEVEL, or by calling the SetLogLevel method of the logger class. The default 
+logging level is 0 (critical log only).
+
 
 .. |apifolder| replace:: ``api`` folder
 .. _apifolder: https://github.com/ROCm/rocDecode/tree/develop/api