Add documentation of new environment variables (#259)

Document SR_THREAD_COUNT and SR_MODEL_TIMEOUT environment variables and update default thread count to 4.
[ committed by @billschereriii  ]
[ reviewed by @ashao  @mellis13 ]

doc/runtime.rst

@@ -107,6 +107,14 @@ The functions for changing this default behavior are:
     The function ``Client.use_model_ensemble_prefix()`` controls
     object prefixing for model and script data.
 
+Model Execution Environment Variable
+====================================
+
+The ``SR_MODEL_TIMEOUT`` environment variable defines a timeout
+on the length of time SmartRedis will wait for a model to
+execute. The value for this variable is measured in milliseconds,
+and the default value is one minute.
+
 Connection and Command Execution Environment Variables
 ======================================================
 
@@ -130,3 +138,14 @@ The user can set these environment variables to adjust command execution behavio
 ``SR_CMD_TIMEOUT`` should be specified in seconds.  Note that ``SR_CMD_INTERVAL``
 and ``SR_CMD_TIMEOUT`` are read during client initialization and not
 before each command execution.
+
+The environment variable ``SR_THREAD_COUNT`` is used by SmartRedis to determine
+the number of threads to initialize when building a worker pool for parallel task
+execution. The default value is four. If the variable is set to zero, SmartRedis
+will use a default number of threads equal to one per hardware context in the
+processor on which the library is running (more specifically, SmartRedis will
+use the result of a call to std::thread::hardware_concurrency() as the number
+of threads to create). This default will generally give good
+performance; however, if the SmartRedis library is sharing the processor hardware
+with other software, it may be useful to specify a smaller number of threads for
+some workloads.

include/redisserver.h

@@ -559,10 +559,8 @@ class RedisServer {
 
         /*!
         *   \brief Default number of threads for thread pool
-        *          (zero, special value that translates to one
-        *          per hardware context)
         */
-        static constexpr int _DEFAULT_THREAD_COUNT = 0;
+        static constexpr int _DEFAULT_THREAD_COUNT = 4;
 
         /*!
         *   \brief Seeding for the random number engine