Add documents for two new environment variables for memory pool.

docs/faq/env_var.md

@@ -58,6 +58,15 @@ $env:MXNET_STORAGE_FALLBACK_LOG_VERBOSE=0
   - Values: Int ```(default=5)```
   - The percentage of GPU memory to reserve for things other than the GPU array, such as kernel launch or cudnn handle space.
   - If you see a strange out-of-memory error from the kernel launch, after multiple iterations, try setting this to a larger value.  
+* MXNET_GPU_MEM_POOL_TYPE
+  - Values: String ```(default=Naive)```
+  - The type of memory pool.
+  - Choices:
+    - Naive: A simple memory pool that allocates memory for the exact requested size and cache memory buffers. If a buffered memory chunk matches the size of a new request, the chunk from the memory pool will be returned and reused.
+    - Round: A memory pool that always rounds the requested memory size and allocates memory of the rounded size. MXNET_GPU_MEM_POOL_ROUND_LINEAR_CUTOFF defines how to round up a memory size. Caching and allocating buffered memory works in the same way as the naive memory pool.
+* MXNET_GPU_MEM_POOL_ROUND_LINEAR_CUTOFF
+  - Values: Int ```(default=24)```
+  - The cutoff threshold that decides the rounding strategy. Let's denote the threshold as T. If the memory size is smaller than `2 ** T` (by default, it's 2 ** 24 = 16MB), it rounds to the smallest `2 ** n` that is larger than the requested memory size; if the memory size is larger than `2 ** T`, it rounds to the next k * 2 ** T.
 
 ## Engine Type