Add class docs for several sensor classes

Various sensor classes added / refactored recently have missing or
out-of-date documentation. Update them to be more in line with what the
classes are doing now.

Bug: 139693714
Test: N/A
Change-Id: Ic791b36eee73b5e969fb2add77c07337828faa9b
diff --git a/core/include/chre/core/sensor_request_manager.h b/core/include/chre/core/sensor_request_manager.h
index 022b43f..0f6f746 100644
--- a/core/include/chre/core/sensor_request_manager.h
+++ b/core/include/chre/core/sensor_request_manager.h
@@ -30,8 +30,13 @@
 
 namespace chre {
 
-// TODO(b/139693714): Update documentation for this and various other sensor
-// classes.
+/**
+ * Handles requests from nanoapps for sensor data and information. This includes
+ * multiplexing multiple requests into one for the platform to handle.
+ *
+ * This class is effectively a singleton as there can only be one instance of
+ * the PlatformSensorManager instance.
+ */
 class SensorRequestManager : public NonCopyable {
  public:
   /**
diff --git a/core/include/chre/core/sensor_request_multiplexer.h b/core/include/chre/core/sensor_request_multiplexer.h
index 4c2d70e..7b02c5d 100644
--- a/core/include/chre/core/sensor_request_multiplexer.h
+++ b/core/include/chre/core/sensor_request_multiplexer.h
@@ -23,7 +23,8 @@
 namespace chre {
 
 /**
- * Keeps track of the state of a sensor along with the various requests for it.
+ * Provides methods on top of the RequestMultiplexer class specific for working
+ * with SensorRequest objects.
  */
 class SensorRequestMultiplexer : public RequestMultiplexer<SensorRequest> {
  public:
diff --git a/core/include/chre/core/sensor_type.h b/core/include/chre/core/sensor_type.h
index 95bef64..59afb1c 100644
--- a/core/include/chre/core/sensor_type.h
+++ b/core/include/chre/core/sensor_type.h
@@ -22,6 +22,15 @@
 
 #include "chre_api/chre/sensor.h"
 
+/**
+ * @file
+ * This file contains miscellaneous types useful in the core framework and
+ * platform implementations for dealing with sensors.
+ *
+ * Additionally, it contains methods useful for converting between these types
+ * and types used by the CHRE APIs and other miscellaneous helper methods that
+ * help make the rest of the codebase more readable.
+ */
 namespace chre {
 
 //! Indicates the reporting mode of the sensor
diff --git a/platform/include/chre/platform/platform_sensor_manager.h b/platform/include/chre/platform/platform_sensor_manager.h
index 604089d..6d4276f 100644
--- a/platform/include/chre/platform/platform_sensor_manager.h
+++ b/platform/include/chre/platform/platform_sensor_manager.h
@@ -24,6 +24,11 @@
 
 namespace chre {
 
+/**
+ * Handles communicating with all CHRE-supported sensors in the system at the
+ * behest of the core framework while also managing the receipt of various
+ * sensor events that CHRE is able to process.
+ */
 class PlatformSensorManager : public PlatformSensorManagerBase {
  public:
   ~PlatformSensorManager();