Fix memory and metrics documentation (facebookincubator#8295)

Summary: Pull Request resolved: facebookincubator#8295

Reviewed By: tanjialiang

Differential Revision: D52605853

Pulled By: xiaoxmeng

fbshipit-source-id: d6cd6b5d2f83604000601bd8fabb0eb6e92cd872

velox/docs/develop/memory.rst

@@ -121,7 +121,7 @@ Memory Manager
     :align: center
     :alt: Memory Manager
 
-The memory manager is created on server startup with provided
+The memory manager is created on server startup with the provided
 *MemoryManagerOption*. It creates a memory allocator instance to manage the
 physical memory allocations for both query memory allocated through memory pool
 and cache memory allocated through the file cache. It ensures the total
@@ -170,8 +170,8 @@ significant memory usage during a system operation, and eventually the memory
 allocator guarantees the actual allocated memory are within the system memory
 limit no matter if it is for system operation or for user query execution. In
 practice, we shall reserve some space from the memory allocator to compensate
-for such system memory usage. We can do that by configuring a smaller query
-memory limit (*MemoryManagerOptions::arbitratorCapacity*) than system memory
+for such system memory usage. We can do that by configuring the query
+memory limit (*MemoryManagerOptions::arbitratorCapacity*) to be smaller than the system memory
 limit (*MemoryManagerOptions::allocatorCapacity*) (refer to `OOM prevention section <#server-oom-prevention>`_
 for detail).
 
@@ -205,18 +205,18 @@ Here is the code block from Prestissimo that initializes the Velox memory system
      ...
    }
 
-* L6: set the memory allocator capacity (system memory limit) from
-  Prestissimo system config
-* L7: set the memory allocator type from Prestissimo system config. If
+* L5: set the memory allocator capacity (system memory limit) from
+  the Prestissimo system config
+* L6: set the memory allocator type from the Prestissimo system config. If
   *useMmapAllocator* is true, we use *MmapAllocator*, otherwise use
   *MallocAllocator*. `Memory Allocator section <#memory-allocator>`_ describes these two
   types of allocators
-* L9: set the memory arbitrator kind from the Prestissimo system config.
+* L8: set the memory arbitrator kind from the Prestissimo system config.
   Currently, we only support the *“SHARED”* arbitrator kind (see `memory arbitrator section <#memory-arbitrator>`_).
   *“NOOP”* arbitrator kind will be deprecated soon (`#8220 <https://github.com/facebookincubator/velox/issues/8220>`_)
-* L11: set the memory arbitrator capacity (query memory limit) from
+* L10: set the memory arbitrator capacity (query memory limit) from the
   Prestissimo system config
-* L14: creates the process-wide memory manager which creates memory
+* L13: creates the process-wide memory manager which creates memory
   allocator and arbitrator inside based on MemoryManagerOptions initialized from previous steps
 * L15-19: creates the file cache if it is enabled in Prestissimo system
   config

velox/docs/metrics.rst

@@ -1,9 +1,9 @@
 
-==============
-Runtime Metric
-==============
+===============
+Runtime Metrics
+===============
 
-Runtime metric is used to collect the metrics of important velox runtime events
+Runtime metrics are used to collect the metrics of important velox runtime events
 for monitoring purpose. The collected metrics can provide insights into the
 continuous availability and performance analysis of a Velox runtime system. For
 instance, the collected data can help automatically generate alerts at an