fix comments

Author: pcmoritz

cpp/src/plasma/client.h

@@ -38,8 +38,9 @@ ARROW_DEPRECATED("PLASMA_DEFAULT_RELEASE_DELAY is deprecated")
 constexpr int64_t kDeprecatedPlasmaDefaultReleaseDelay = 64;
 #define PLASMA_DEFAULT_RELEASE_DELAY kDeprecatedPlasmaDefaultReleaseDelay;
 
-/// Number of objects that will be kept unreleased in the plasma client
-/// until we send a message to the store to release the object.
+/// We keep a queue of unreleased objects cached in the client until we start
+/// sending release requests to the store. This is to avoid frequently mapping
+/// and unmapping objects and evicting data from processor caches.
 constexpr int64_t kPlasmaDefaultReleaseDelay = 64;
 
 /// Object buffer data structure.

cpp/src/plasma/format/plasma.fbs

@@ -18,6 +18,7 @@
 // Plasma protocol specification
 
 enum MessageType:int {
+  // Message that gets send when a client hangs up.
   PlasmaDisconnectClient = 0,
   // Create a new object.
   PlasmaCreateRequest,

cpp/src/plasma/io.cc

@@ -30,7 +30,7 @@ using arrow::Status;
 
 /// Number of times we try connecting to a socket.
 constexpr int64_t kNumConnectAttempts = 50;
-/// Time to wait between connection attempts.
+/// Time to wait between connection attempts to a socket.
 constexpr int64_t kConnectTimeoutMs = 100;
 
 namespace plasma {